understanding api icp653 reading 4 api651

Readings on API ICP 653API651 Cathodic ProtectionMy Pre-exam Self Study Notes9th January 2016

Charlie Chong/ Fion Zhang


Petroleum Tank


Petroleum Tanks

API 653 Exam Administration -- Publications Effectivity Sheet FOR: November 2015, March 2016 and July 2016 Listed below are the effective editions of the publications required for this exam for the date(s) shown above. API Recommended Practice 571, Damage Mechanisms Affecting Fixed Equipment in the Refining Industry, Second Edition, April 2011


ATTENTION: Only the following sections / mechanisms from RP 571 are included on the exam:

Section 3, Definitions Par. 4.2.7 Brittle Fracture 4.2.16 Mechanical Fatigue 4.3.2 Atmospheric Corrosion 4.3.3 Corrosion Under insulation (CUI) 4.3.8 Microbiologically Induced Corrosion (MIC) 4.3.9 Soil Corrosion 4.3.10 Caustic Corrosion 4.5.1 Chloride Stress Corrosion Cracking (Cl-SCC) 4.5.3 Caustic Stress Corrosion Cracking (Caustic Embrittlement) 5.1.1.10 Sour Water Corrosion (Acidic) 5.1.1.11 Sulfuric Acid Corrosion



• API Recommended Practice 575, Inspection of Atmospheric and Low-Pressure Storage Tanks, Third Edition, April 2014

• API Recommended Practice 577 – Welding Inspection and Metallurgy, Second Edition, December 2013

• API Standard 650, Welded Tanks for Oil Storage, Twelfth Edition, March 2013 with Addendum 1 (September 2014), Errata 1 (July 2013), andErrata 2 (December 2014).

• API Recommended Practice 651, Cathodic Protection of AbovegroundPetroleum Storage Tanks, Fourth Edition, September 2014.

• API Recommended Practice 652, Lining of Aboveground Petroleum Storage Tank Bottoms, Fourth Edition, September 2014

• API Standard 653, Tank Inspection, Repair, Alteration, and Reconstruction, Fifth Edition, November 2014.


• American Society of Mechanical Engineers (ASME), Boiler and Pressure Vessel Code, 2013 Edition i. ASME Section V, Nondestructive Examination, Articles 1, 2, 6, 7 and

23 (section SE-797 only) ii. Section IX, Welding and Brazing Qualifications (Welding Only)

See end of this study note for API Official BOK


ASTM Mentioned- mentioned in this standard

ASTM G57- Standard Test Method for Field Measurement of Soil Resistivity Using the Wenner Four-Electrode MethodASTM G51- Standard Test Method for Measuring pH of Soil for Use in Corrosion TestingASTM D512- Standard Test Methods for Chloride Ion In WaterASTM D516- Standard Test Method for Sulfate Ion in WaterASTM D1557- Standard Test Methods for Laboratory Compaction Characteristics of Soil Using Modified Effort (56,000 ft-lbf/ft3 (2,700 kN-m/m3))


Keywords: Pitting corrosion on steel can begin at chloride levels of 10 ppm. Sulfate levels > 200 ppm frequently indicate high concentrations of organic

matter. Sulfide levels > 0.10 ppm, may indicate that sulfates have been reduced

by bacteria. (SRB) The corrosion rate of steel may double with every 18 °F (10 °C) increase in

temperature above 77 °F (25 °C). Galvanic systems are normally applied only to small diameter tanks [e.g.

less than 20 ft (7 m)] or for tanks with externally coated bottoms. Because of decreased life at higher temperatures, selenium rectifiers are

not recommended if ambient temperatures are expected to exceed 130 °F (55 °C).


Keywords: Galvanic cathodic protection systems may be more economical on small

diameter tanks [less than 20 ft (6 m); see NACE RP0193 and 6.2 of this RP].

current requirement test, the generally accepted protective current density is between 1 mA/ft2 and 2 mA/ft2 at ambient temperatures (0.5 mA/ft2 for close coupled anodes), and current density between 2 mA/ft2 and 8 mA/ft2for elevated temperature tanks.

http://independent.academia.edu/CharlieChong1http://www.yumpu.com/zh/browse/user/charliechonghttp://issuu.com/charlieccchong


http://greekhouseoffonts.com/Charlie Chong/ Fion Zhang


The Magical Book of Tank Inspection ICP


闭门练功

闭门练功

Charlie C

hong/ Fion Zhang


闭门练功10 Jane2016

Fion Zhang at Shanghai9th January 2016



Cathodic Protection of Aboveground Petroleum Storage TanksAPI Recommended Practice 651Fourth Edition, September 2014


Cathodic Protection of Aboveground Petroleum Storage Tanks1 Scope1.1 The purpose of this recommended practice (RP) is to present procedures and practices for achieving effective corrosion control on aboveground storage tank bottoms through the use of cathodic protection. This RP contains provisions for the application of cathodic protection to existing and new aboveground storage tanks. Corrosion control methods based on (1) chemical control of the environment or (2) the use of protective coatings are not covered in detail.


1.2 When cathodic protection is used for aboveground storage tank applications, it is the intent of this RP to provide information and guidance specific to aboveground metallic storage tanks in hydrocarbon service. Certain practices recommended herein may also be applicable to tanks inother services. It is intended to serve only as a guide to persons interested incathodic protection. Specific cathodic protection designs are not provided.Such designs should be developed by a person thoroughly familiar withcathodic protection practices for aboveground petroleum storage tanks.

1.3 This RP does not designate specific practices for every situation because the varied conditions in which tank bottoms are installed preclude standardization of cathodic protection practices.


2 Normative References2.1 Standards, Codes, Publications, and SpecificationsThe following referenced documents are indispensable for the application of this document. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments) applies.


API 12A Specification for Oil Storage Tanks with Riveted Shells Last Print Edition 1951API Specification 12B, Bolted Tanks for Storage of Production LiquidsAPI Std 12C:1955 API specification for welded oil storage tanks. - 13th editionAPI Specification 12D, Field Welded Tanks for Storage of Production LiquidsAPI 12E is a code standard on WOODEN PRODUCTION TANKS with last edition of API 12E in 1956API Specification 12F, Shop Welded Tanks for Storage of Production Liquids

Note:A- RivetedB- BoltedC- WeldedD- Filed weldedE- WoodenF- Shop welded


• API Recommended Practice 500, Recommended Practice for Classification of Locations for Electrical Installations at Petroleum Facilities

• API Recommended Practice 575, Inspection of Atmospheric and Low Pressure Storage Tanks

• API Standard 620, Design and Construction of Large, Welded, Low-pressure Storage Tanks

• API Standard 650, Welded Tanks for Oil Storage• API Recommended Practice 652, Lining of Aboveground

PetroleumStorage Tank Bottoms• API Standard 653, Tank Inspection, Repair, Alteration, and Reconstruction• API Recommended Practice 1615, Installation of Underground Petroleum

Storage Systems• API Recommended Practice 1621, Bulk Liquid Stock Control at Retail

Outlets• API Recommended Practice 1632, Cathodic Protection of Underground

Petroleum Storage Tanks and Piping Systems


• API Recommended Practice 575, Inspection of Atmospheric and Low Pressure Storage Tanks

• API Standard 653, Tank Inspection, Repair, Alteration, and Reconstruction

• API Recommended Practice 1632, Cathodic Protection of Underground Petroleum Storage Tanks and Piping Systems

• Api Recommended Practice 651 Cathodic Protection of AbovegroundPetroleum Storage Tanks


• API Recommended Practice 2003, Protection Against Ignitions Arising Out of Static, Lightning, and Stray Currents

• ASTM C144 1, Standard Specification for Aggregate for Masonry Mortar• ASTM C778, Standard Specification for Standard Sand• ASTM D512, Standard Test Methods for Chloride Ion in Water• ASTM D516, Standard Test Method for Sulfate Ion in Water• ASTM D1557, Standard Test Methods for Laboratory Compaction

Characteristics of Soil Using Modified Effort (56,000 ft-lbf/ft3 (2700 kN-m/m3))

• ASTM G51, Standard Test Method for Measuring pH of Soil for Use in Corrosion Testing

• ASTM G57, Method for Field Measurement of Soil Resistivity Using the Wenner Four-Electrode Method

• EPA 0376.1 2, Test Method for Sulfide—Titrimetric Iodine


• NACE, Peabody’s Control of Pipeline Corrosion, ISBN 1-57590-114-5• NACE TM0497, Measurement Techniques Related to Criteria for Cathodic

Protection on Underground or Submerged Metallic Piping Systems• NACE SP0177-2007, Mitigation of Alternating Current and Lightning

Effects on Metallic Structures and Corrosion Control Systems• NACE RP0193-2001, External Cathodic Protection of On-Grade Metallic

Storage Tank Bottoms• NACE SP0285-2011, Corrosion Control of Underground Storage Tank

Systems by Cathodic Protection• NACE SP0388-2007, Impressed Current Cathodic Protection of Internal

Submerged Surfaces of Steel Water Storage Tanks• NACE SP0572-2007, Design, Installation, Operation, and Maintenance of

Impressed Current Deep Groundbeds• NACE SP0575-2007, Internal Cathodic Protection Systems in Oil Treating

Vessels• NACE TPC 11, A Guide to the Organization of Underground Corrosion

Control Coordinating Committees


• NFPA 30, Flammable and Combustible Liquids Code• NFPA 70, National Electrical Code• PEI RP100, Recommended Practices for the Installation of Underground

Liquid Storage Systems

1. ASTM International, 100 Bar Harbor Drive, West Conshohocken, Pennsylvania 19428, www.astm.org.

2. U.S. Environmental Protection Agency, Ariel Rios Building, 1200 Pennsylvania Avenue, N.W., Washington, D.C. 20460, www.epa.gov.

3. NACE International (formerly the National Association of Corrosion Engineers), 1440 South Creek Drive, Houston, Texas, 77084-4906, www.nace.org.

4. National Fire Protection Association, 1 Batterymarch Park, Quincy, Massachusetts 02169-7471, www.nfpa.org.

5. Petroleum Equipment Institute, P.O. Box 2380, Tulsa, Oklahoma 74101-2380. www.pei.org.

6. Underwriters Laboratories, 333 Pfingsten Road, North brook, Illinois 60062, www.ul.com.


2.2 Other ReferencesAlthough not cited in the text of this RP, the following publications provide additional information pertaining to cathodic protection of abovegroundstorage tanks in petroleum service.■ API Standard 2610, Design, Construction, Operation, Maintenance and

Inspection of Terminal & Tank Facilities■ UL 142, Steel Aboveground Tanks for Flammable and Combustible

Liquids


3 Terms and DefinitionsDefinitions in this section reflect the common usage among practicing corrosion control personnel. In many cases, in the interests of brevity and practicality, the strict scientific definitions have been abbreviated or paraphrased. For the purposes of this document, the following definitions apply.3.1 aboveground storage tankAn on-grade, stationary, uniformly supported container, usually cylindrical in shape, consisting of a metallic roof, shell, bottom, and support structure where more than 90 % of the tank volume is above surface grade.3.2 anodeThe electrode of an electrochemical cell at which oxidation (corrosion) occurs. Antonym: cathode.3.3 anode bedConsists of one or more anodes installed below the earth’s surface for the purpose of supplying cathodic protection.


Anode/ Cathode Reactions

http://chemwiki.ucdavis.edu/Analytical_Chemistry/Electrochemistry/Case_Studies/Corrosion/Corrosion


3.4 backfillCommercial, uniformly conductive material placed in a hole to fill the space around anodes, vent pipe, and buried components of a cathodic protection system. Anodes can be prepackaged with backfill material for ease ofinstallation.3.5 breakout piping/tanksPiping/tanks used to relieve surges in a hazardous liquid pipeline system or piping/tanks used to receive and store hazardous liquid transported by a pipeline for re-injection and continued transportation by pipeline.3.6 cathodeThe electrode of an electrochemical cell at which a reduction reaction occurs. Antonym: anode.3.7 cathodic protectionA technique to reduce corrosion of a metal surface by making the entire surface the cathode of an electrochemical cell.


3.8 chime (or chine)The portion of the tank bottom steel floor plate that extends horizontally past the outside vertical surface of the shell (i.e. the external lip formed at the base of the tank where the bottom steel floor plate protrudes and is welded to the bottom of the shell, around the entire tank perimeter). Also referred to as bottom extension.


3.9 coke breezeA commercial carbonaceous backfill material. (around anode)3.10 concentration corrosion cellA form of localized corrosion initiated by the difference in metal ion or oxygen concentration due to (1) crevices, (2) deposits, or (3) differences in oxygen concentration on the tank bottom. (NACE definition: An electrochemical cell, the electromotive force of which is caused by a difference in concentration of some component in the electrolyte. [This difference leads to the formation of discrete cathodic and anodic regions.]) 3.11 continuity bond A metallic connection that provides electrical continuity. 3.12 corrosionThe deterioration of a material (chemical, mechanical, etc.), usually a metal, that results from a reaction with its environment.3.13 current densityThe current per unit area flowing to or from a metallic surface.


3.14 current requirement testA test where the use of direct current flow from a temporary anode bed to the structure to be protected is used to determine the amount of current necessary to protect the structure.3.15 deep anode bedOne or more anodes installed vertically at a nominal depth of 15 m (50 ft) or more below the earth’s surface in a single drilled hole for the purpose of supplying cathodic protection.

15 m (50 ft) or more


deep anode bedOne or more anodes installed vertically at a nominal depth of 15 m (50 ft) or more below the earth’s surface in a single drilled hole for the purpose of supplying cathodic protection.

m

Charlie C

hong/ Fion Zhang

deep anode bedOne or more anodes installed vertically at a nominal depth of 15 m (50 ft) or more.


3.16 differential aeration cellAn electrochemical cell the electromotive force of which is due to a difference in air (oxygen) concentration at one part of the tank bottom as compared with that at another part of the tank bottom of the same material.3.17 electrical isolationThe condition of being electrically separated from other metallic structures.


Differential aeration cell


3.18 electrochemical cellAn electrochemical system consisting of an anode and a cathode immersed in an electrolyte so as to create an electrical circuit. The anode and cathodemay be separate metals or dissimilar areas on the same metal. The cellincludes the external circuit, which permits the flow of electrons from theanode toward the cathode. (anode, cathode and external circuit)3.19 electrode potential The potential of an electrode as measured against a standard reference electrode. The electrode potential does not include any voltage measurement errors due to the passage of current through the resistance of the metallic or electrolytic paths.3.20 electrolyte A chemical substance containing ions that migrate in an electric field. For the purposes of this RP, electrolyte refers to the soil or water adjacent to and in contact with the bottom of an aboveground petroleum storage tank, including the contaminants and chemicals contained therein.


Electrode Potential: Standard Electrode: H+


Electrode Potential: Standard Electrode: H+

http://www.substech.com/dokuwiki/doku.php?id=electrode_potentials


Electrode Potential: Standard Electrode: AgCl-

http://www.ijcambria.com/ALS_Reference_Electrodes.htm


Electrode Potential: Cu/ CuSO4 Reference Electrode


Electrode Potential: CP- Test Point

https://www.corrosionpedia.com/2/2494/corrosion-prevention/cathodic-protection/an-overview-of-cathodic-protection-potential-measurement

Charlie Chong/ Fion Zhang http://www.aiworldwide.com/products/bullhorn/rm415x/


3.21 environmental crackingThe brittle fracture of a normally ductile material in which the corrosive effect of the environment is a causative factor.3.22 external circuitConsists of the wires, metallic connectors, measuring devices, current sources, etc., that are used to bring about or measure the desired electrical conditions within an electrochemical cell. It is this portion of the cell through which electrons travel.3.23 external linerA system or device, such as a non-conductive membrane, installed beneath an aboveground storage tank, in or on the tank dike, to contain any accidentally escaped product. (RPB)3.24 foreign structureAny metallic structure that is not intended as a part of a system under cathodic protection.


There are several practical techniques that can be used for potential measurements. They are very important for any structure-to-electrolyte potential survey. Practical Techniques for Cathodic Protection Potential Measurement

In order to determine the adequacy of a cathodic protection system applied on a certain structure, its polarized potential has to be measured. The polarized potential is the summation of corrosion potential and the amount of polarization that the structure has, excluding the soil IR drop. Polarized potential is the potential across the structure-to-electrolyte interface. Below, we will talk about the practical techniques that can be used to measure a structure's polarized potential.

https://www.corrosionpedia.com/2/3067/corrosion-prevention/cathodic-protection/practical-techniques-for-cathodic-protection-potential-measurement


1. Current Interruption TechniqueThe soil IR drop can be excluded from the measured potential by interrupting the cathodic protection (CP) current source instantaneously. This can be done by installing a current interrupter at the CP source and adjusting the ON and OFF cycles. One should make sure that the OFF cycle is as short as possible to avoid structure depolarization, yet long enough to be able for reading records.

It's important to know that this technique is very simple for application in limited systems, but it faces many challenges when applied in large networks. These challenges include:


Positive SpikingWhen the current is interrupted, a positive spike occurs in the measured potential due to the inductive and capacitive effects of the pipeline, which doesn't represent the true potential of the pipeline. The duration of this spike may be 0.3 seconds. So, the instant OFF potential reading, which represents the polarized one, should be recorded after this time has passed.


Interruption of all Current SourcesThis happens in large networks of pipelines, where there are many current sources distributed at different sections of the network, as well as in urban areas, where pipelines of different authorities run with a lot of parallelism and crossings—and so stray currents exist in pipelines. Current interrupters should be installed at all CP sources and synchronized so that the CP current can be interrupted at the same time and then all IR drops can be eliminated. When synchronization of current interrupters can't be done, the total IR drop can be computed by taking the summation of individual IR drops. Synchronization of current interrupters can be achieved by using GPS-synchronized interrupters, which take the same signal from the satellite. However, if signal is lost at any interrupter, the measured potential will not represent the true one.


2. Reference Electrode near the Structure

By placing the reference electrode as close as possible to the protected structure, the soil IR drop can be minimized. The reference electrode should not be too close to the structure so that it doesn't eliminate the CP current; (?)it is recommended to be at a distance of twice the reference electrode diameter.

This technique is impractical for buried pipelines, except at places where the pipeline enters or exits from the ground. This technique can be used for underwater pipelines.

For coated pipelines, if the reference electrode is placed close to the pipeline and near the coating, the IR drop can't be minimized. The reference electrode would need to be placed close to the coating holidays for accurate measurements, which is not practical. (?)


Inside water storage tanks, the electrode should be positioned as close to the wall of the tank as possible. The same is true for waterfront and offshore structures; the electrode should be as close to the piling as possible. In moving water, the electrode may swing about, so some structures are equipped with guide wires or perforated plastic ducts to restrict the movement of a portable electrode.For on-grade storage tanks, data are frequently taken around the periphery of the tank. This may not yield accurate data about the potentials under the tank bottom, particularly if the anodes are in a ring around the tank or the tank is large in diameter. Stationary reference electrodes under the tank bottom yield the best data. Alternatively, if a perforated plastic tube is installed under the tank and filled with water, a reference electrode can be pulled through it and potentials measured at intervals underneath.An alternative to placing the reference electrode close to the structure is to install a plastic tube filled with soil from the grade next to the pipe surface and put the reference electrode in it.


3. External CP CouponsCP coupons are intended to simulate a small portion of a well-coated pipeline like a holiday; they are manufactured from the same alloy as the protected structure, and they are typically 10 to 100 cm² in surface area.CP coupons should be buried near the protected structure in the same electrolyte, subjected to the same CP current and connected electrically to the protected structure.


CP coupons can be used to determine the corrosion rate of the structure or to monitor the adequacy of the applied CP system.In order to determine the corrosion rate, the coupon needs to be weighed beforehand and then buried and connected to the structure. After a certain time, the coupons should be removed and weighed. The corrosion rate is the weight loss per time.For monitoring of CP potential, the connection between the coupon and the pipeline needs to be interrupted instantaneously and the reference electrode should be placed in a soil tube to eliminate any IR drop in the soil. So, the polarized potential of the coupon can be measured with respect to the reference electrode placed in the soil tube. If the polarized potential of the coupon is -850 mv w.r.t cse (Cu/CuSO4) or more negative, any holiday of the same size or smaller will be equally well protected.The above techniques are the most practical ones used for potential measurements, but in order to obtain correct measurements, proper instruments have to be used. These techniques are very important for any structure-to-electrolyte potential survey.


3.25 galvanic anodeA metal that, because of its relative position in the galvanic series, provides sacrificial protection to another metal that is more noble, when electrically coupled in an electrolyte. These anodes are the source of current in galvanic or sacrificial cathodic protection.3.26 galvanic cathodic protectionThe reduction or prevention of corrosion of a metal in an electrolyte by electrically connecting it to a more anodic metal.3.27 galvanic seriesA list of metals and alloys arranged according to their relative potentials in a given environment.3.28 holidayA discontinuity in a protective coating that exposes the underlying substrate to the environment.


3.29 impressed currentAn electric current supplied by a device employing a power source that is external to the electrode system. (An example is the direct current output of a cathodic protection rectifier.)3.30 interference bondA metallic connection designed to control stray electrical current discharge from a metallic structure.3.31 IR dropThe voltage generated across a resistance by an electrical current in accordance with Ohm’s Law: E = I × R. For the purpose of this RP, the most significant IR drop is the portion of a structure-to-soil potential caused by current flow through a resistive electrolyte from the anode to the structure.(resistive coating to the electrolyte?)

Rsoil

Rcoating


3.32 isolationSee electrical isolation.3.33 membraneA thin, continuous sheet of nonconductive synthetic material used to contain and/or separate two different environments.3.34 oxidationThe loss of electrons by a constituent of a chemical reaction.(e.g. Fe→ Fe2+ + 2e- )


3.35 polarizationThe change from the open-circuit potential of an electrode as a result of current across the electrode/electrolyte interface.3.36 rectifierA device for converting alternating current to direct current. Usually includes a step-down AC transformer, a silicon or selenium stack (rectifying elements), meters, and other accessories when used for cathodic protection purposes.3.37 reductionThe gain of electrons by a constituent of a chemical reaction.(e.g. O2 + 2H2O + 4e- → 4OH-)(e.g. 2H+ + 2e- → H2↑)


3.38 reference electrodeAn electrode whose open-circuit potential is constant under similar conditions of measurement.

http://chemwiki.ucdavis.edu/Analytical_Chemistry/Analytical_Chemistry_2.0/11_Electrochemical_Methods/11B%3A_Potentiometric_Methods


Schematic diagram showing the saturated calomel electrode.



Schematic diagram showing a Ag/AgCl electrode. Because the electrode does not contain solid KCl, this is an example of an unsaturated Ag/AgCl electrode.



Reference Electrode

http://1001italia.org/news/ph-meter-electrode-function.html


Reference Electrode

http://www.chem1.com/acad/webtext/elchem/ec2.html


Potentials of Common Reference Electrodes

http://www.consultrsr.net/resources/ref/refpotls.htmhttp://wiki.edaq.com/index.php/Reference_Electrode_Potentials


More on Nernst Equation Exerciseshttp://chemwiki.ucdavis.edu/Analytical_Chemistry/Analytical_Chemistry_2.0/11_Electrochemical_Methods/11B%3A_Potentiometric_Methods


3.39 release prevention barrier (RPB)Includes steel bottoms (when used in a double bottom or secondary containment system), synthetic materials, clay liners, and all other barriers or combination of barriers placed in the bottom of, or under an aboveground storage tank, which have the following functions: (a) preventing the escape ofstored product, and (b) containing or channeling released material for leak detection.3.40 resistorA device used within an electrical circuit to control current flow.3.41 secondary containmentA device or system used to control the accidental escape of a stored product so it may be properly recovered or removed from the environment.


3.42 shallow anode bedA group of cathodic protection anodes installed individually, spaced uniformly, and typically buried less than 20 ft (6 m) below grade.3.43 shuntA conductor of a known electrical resistance through which current flow may be determined by measurement of the voltage across the conductor and calculation using Ohm’s Law.3.44 stray currentCurrent flowing through paths other than the intended circuit.3.45 stray current corrosionCorrosion resulting from direct current flow through paths other than the intended circuit.3.46 stress corrosion crackingThe fracture of a metal by the combined action of corrosion and tensile stress. The fracture occurs under a tensile stress that may be well below the tensile strength or even the yield strength of the material.


3.47 structure-to-electrolyte voltage (also structure-to-soil potential or pipe-to-soil potential)The voltage difference between a metallic structure and the electrolyte which is measured with a reference electrode in contact with the electrolyte.The voltage difference between (1) a metallic structure and (2) a reference electrode in contact with the electrolyte. (?)3.48 structure-to-structure voltage (also structure-to structure potential difference)The difference in voltage between metallic structures in a common electrolyte.3.49 tank padThe material immediately adjacent to the exterior steel bottom of an aboveground storage tank.3.50 test leadAn electrically conductive wire or cable attached to a structure and terminated in a test station. It is used for the measurement of structure-to-electrolyte potentials and other measurements.


3.51 test stationA small enclosure or housing that is the termination point of one or more test leads.3.52 voltageAn electromotive force, or a difference in electrode potentials expressed in volts. Also known as a potential.3.53 water bottomA water layer in the bottom of a tank caused by separation of water and product due to differences in solubility and specific gravity.


4 Corrosion of Aboveground Steel Storage Tanks4.1 Introduction4.1.1 Corrosion may be defined as the deterioration of a metal resulting from a reaction with its environment. Corrosion of steel structures is anelectrochemical process. For the corrosion process to occur, areas withdifferent electrical potentials must exist on the metal surface. These areasmust be electrically connected and in contact with an electrolyte.

There are four components required for a corrosion cell: an anode, a cathode, a metallic path connecting the anode and cathode, and an electrolyte (see Figure 1). The role of each component in the corrosion cell is as follows:


a) At the anode, the base metal goes into solution (corrodes) by releasing electrons and forming positive metal ions. For steel, the anodic reaction is:

Fe→ Fe+2 + 2e–

b) At the cathode, chemical reactions take place using electrons released at the anode. No corrosion takes place at the cathode. One common cathodic reaction is:

O2 + 2H2O + 4e– →4OH–


c) The metallic path provides a way for electrons released at the anode to flow to the cathode.

d) The electrolyte contains ions and conducts current from the anode to the cathode by ionic movement. The electrolyte contains both negatively charged ions called anions and positively charged ions called cations that are attracted to the anode and cathode, respectively. Moist soil is the most common electrolyte for external surfaces of the tank bottom, while water and sludge generally are the electrolytes for the internal surfaces.

Keywords:positively charged ions called cationsnegatively charged ions called anions


Figure 1- Electrochemical Corrosion Cell

e-

Fe→ Fe+2 + 2e–O2 + 2H2O + 4e– →4OH–


4.1.2 There are many forms of corrosion. The two most common types relative to tank bottoms are general and localized (pitting) corrosion. Ingeneral corrosion, thousands of microscopic corrosion cells occur on an areaof the metal surface resulting in relatively uniform metal loss. In localized(pitting) corrosion, the individual corrosion cells are larger and distinct anodicand cathodic areas can be identified. Metal loss in this case may beconcentrated within relatively small areas with substantial areas of thesurface unaffected by corrosion.

4.1.3 The composition of the metal is a factor in determining which areas on a metal surface become anodes or cathodes. Differences in electrochemical potential between adjacent areas can result from uneven distribution of alloying elements or contaminants within the metal structure. Corrosion can also be caused by differences between weld metal, heat affected zone, and parent metal.

Comments:Potential difference may be caused by: Metallurgical, stress levels, different aeration, different electrolytes, temperature differences, stray current.


4.1.4 Physical and chemical properties of the electrolyte also influence the location of cathodic and anodic areas on the metal surface. For example,differing oxygen concentrations between areas on a steel surface maygenerate differences in potential. Areas with lower oxygen concentrationsbecome anodic and areas with higher oxygen concentrations becomecathodic. This phenomenon can cause corrosion of steel tank bottoms on ahomogeneous sand pad, and/or when contaminated with clay or other debrisor natural soil that does not have a uniform consistency (see Figure 2).

4.1.5 Soil characteristics substantially affect the type and rate of corrosion on a structure in contact with soil. For example, dissolved salts influence thecurrent carrying capacity of the soil electrolyte and help determine reactionrates at the anodic and cathodic areas. Moisture content, pH, oxygenconcentration, and other factors interact in a complex fashion to influencecorrosion.


Figure 2- Oxygen Concentration Cell Caused by Rocks or Clay in Tank Pad


4.2 Corrosion Mechanisms4.2.1 Stray Current CorrosionStray currents (also known as interference currents) travel through the soil electrolyte and on to structures for which they are not intended. Usually, theaffected structure collects the interference currents from the electrolyte; Thesource of these currents is not electrically connected to the affected structure.As shown in Figure 3, stray current may enter an unprotected tank bottomand travel through the low resistance path of the metal to an area on the tankcloser to the protected structure (pipeline). At this location, the currentdischarges back into the electrolyte (soil) at point B with resultant metal loss.The most common, and potentially the most damaging, stray currents aredirect currents. These currents are generated from grounded DC electricpower systems including electric railroads, subways, welding machines,impressed current cathodic protection systems, and thermoelectric generators.


Figure 3- Example of Stray Current Corrosion of an Unprotected Tank Bottom


The severity of corrosion (metal loss) resulting from interference currents depends on several factors:

a) separation and routing of the interfering and affected structures and location of the interfering current source;

b) magnitude and density of the current;c) quality or absence of a coating on the affected structures;d) the presence and location of mechanical joints having high electrical

resistance;e) temperature.

Further information related to proper design to avoid stray currents, and detection and control of stray current corrosion, can be found in Section 7 and Section 10.


4.2.2 Galvanic CorrosionGalvanic corrosion occurs when two metals with different compositions (thus, different electrolytic potentials) are connected in an electrolyte (usually soil).Current will flow from the more active metal (anode) to the less active metal(cathode) with accelerated attack at the anodic metal. For example, galvaniccorrosion can occur when a bronze check valve is joined to carbon steelpiping or where stainless steel, copper pipe or a copper ground rod isconnected to a carbon steel tank. In the pipe/steel tank example, the stainlesssteel, copper pipe, or copper ground rod becomes the cathode and the steeltank is the anode. Since current takes the path of least resistance, the mostsevere corrosion attack will occur in the area on the steel tank immediatelyadjacent to the stainless steel or copper pipe as shown in Figure 4.


Figure 4 - Galvanic Corrosion

e-

e-


The extent of such a problem is dependent on several factors. The mostsignificant factors are:

a) the relative surface areas of the cathode and anode;b) the relative potential difference between the two materials as determined

by their position in the galvanic series; Andc) temperature.


The Temperature


4.3 Internal CorrosionExperience has revealed that corrosion may occur on the inside surface of a tank bottom. The extent or nature of corrosion depends on many factors associated with the composition of the fluid in contact with the steel bottom.Major factors which influence the severity of corrosion include:

a) conductivity (a function of dissolved solids);b) suspended solids;c) pH level;d) dissolved gases such as CO2, H2S, or O2;e) sulfate reducing bacteria; (SRB)f) temperature.

Three major types of corrosion to be considered are general corrosion, localized (pitting) corrosion, and to a lesser extent in tanks, environmental cracking. For further discussion of internal corrosion mechanisms, see API 652.


5 Determination of Need for Cathodic Protection5.1 IntroductionThe need for cathodic protection shall be determined for all aboveground storage tanks. This section discusses parameters for consideration whendetermining whether a steel aboveground storage tank bottom requirescathodic protection. If it is determined that corrosion may occur, adequatecorrosion control procedures should be adopted to ensure metal integrity forsafe and economical operation over the service life of the tank. The locationof a facility or the presence of a leak detection system alone should not beused to determine the need for cathodic protection. The decisions governingthe need for cathodic protection should be based on data from inspectionsperformed in accordance with API 653, corrosion surveys; operating records;prior test results with similar tank systems in similar environments; national,state, and local code requirements; and the recommendations made withinthis document.


Due to the wide variety of tank pad types and conditions, and their variability in preventing corrosion with or without cathodic protection, the practices of this standard should be used in conjunction with the inspection and maintenance practices of API 653 (including measuring the electrical and chemical properties of the tank pad) to determine that external corrosion of tank bottoms is being adequately controlled and will continue to be controlled over the long term.


5.1.1 New Aboveground Storage TanksCorrosion control by cathodic protection for new aboveground storage tanks should be included in the initial design unless a detailed study indicates it is not needed. If cathodic protection is applied, it should be maintained during the service life of the tank.

5.1.2 Existing Aboveground Storage TanksAn evaluation should be conducted in accordance with API Standard 653 to determine the need to install cathodic protection. When these studies indicate that corrosion will affect the safe or economic operation of the tank, adequatecorrosion control measures should be installed. These corrosion control measures can include cathodic protection and linings (see API 652).


5.1.3 Internal Cathodic ProtectionPure hydrocarbon fluids are usually not corrosive and do not require corrosion control for internal surfaces. However, based upon experience,internal corrosion will occur in aboveground storage tanks that have internalsurfaces exposed to water, sediments, or other contaminants. Generally,coatings are used to reduce or eliminate corrosion on internal surfaces. Fortanks in petroleum service, internal cathodic protection in conjunction withcoatings can be used and it can be effective in protecting against corrosion atholidays in the coating. For more detailed information on internal cathodicprotection, see NACE SP0575 and SP0388.

Keypoints:NACE SP0575-2007 Internal Cathodic Protection (CP) Systems in Oil-Treating Vessels

NACE SP0388-2014 Standard Practice - Impressed Current Cathodic Protection of Internal Submerged Surfaces of Carbon Steel Water Storage Tanks


5.1.4 Limitations of External Cathodic ProtectionCathodic protection is an effective means of corrosion control only if it is

possible to pass electrical current between the anode and cathode (tank bottom). Many factors can either reduce or eliminate the flow of electrical current and, therefore, may limit the effectiveness of cathodic protection insome cases or preclude its use in others. Such factors include:

a) tank pads such as concrete;b) a non-conductive external liner between the tank bottom and anodes

(unless anodes are installed between liner and tank bottom);c) high resistivity soil or coarse/large rock aggregate pads;d) old aboveground storage tank bottoms left in place when a new bottom is

installed.

These and other related factors are discussed in more detail in 5.3 and 5.4. It should be recognized that external cathodic protection has no effect on internal corrosion.


5.2 Tank History5.2.1 GeneralBefore determining the need for cathodic protection, a full evaluation of tank history is advised. If this evaluation indicates that external corrosion is a known or potential concern, then cathodic protection or other corrosion control measures should be used. If internal corrosion is known to be a problem, use of a lining should be considered (refer to API 652); in certain cases, internal cathodic protection in conjunction with a lining may be appropriate. The following items should be investigated and determined.


5.2.2 Tank Design/Construction HistoryThe following items should be investigated and determined in the evaluation of tank design/construction history:a) ringwall foundation and tank pad design;b) site plan, including tank farm layout;c) construction dates;d) soil electrical and chemical properties;e) water table;f) presence and type(s) of coatings or linings;g) previous repairs;h) change in soil conditions;i) conductive and non-conductive release prevention barriers; (RPB)j) secondary bottom;k) existing cathodic protection on nearby structures;l) maintenance history;m) expected life;n) rectifier and anode bed location(s);o) tank bottom design, coned up, coned down, “w” type, or flat bottom.


5.2.3 Type of ServiceThe following items should be investigated and determined in the evaluation of types of service:a) type of product stored;b) product temperature;c) presence and depth of water bottoms;d) frequency of fill and discharge.


5.2.4 Inspection/Corrosion HistoryThe following items should be investigated and determined in the evaluation of inspection/corrosion history:a) tank inspections per API 653;b) corrosion rate records;c) corrosion problems on nearby tanks;d) corrosion problems on tanks of similar construction;e) stray current problems;f) design and performance of previous corrosion protection systems;g) structure-to-soil potential surveys;h) tank pad chemical and electrical testing during internal inspections.


5.2.5 Other FactorsThe following items should be investigated and determined in the evaluation of other factors:a) foreign buried metallic structures;b) foreign cathodic protection systems.


5.3 Tank Pad and Soil Conditions5.3.1 Introduction5.3.1.1 Different types of pads are constructed for aboveground storage tanks due to a wide variety of surface, subsurface, and climatic conditions. The padmaterial under the tank has a significant effect on external corrosion of thetank bottom and can influence the effectiveness and applicability of externalcathodic protection. It is very important to ensure that there is no debris suchas, rocks, lumps of clay, welding electrodes, paper, plastic, wood, etc. in thepad material. The pad material particles should be fine and uniform.


Tanks should be built on an elevated berm to allow adequate drainage away from the tank bottom. The use of fine particles will provide a denser pad tohelp reduce the influx and outflow of oxygen from the perimeter of the tank asit is emptied and filled. If large particle sizes are used, differential aerationcorrosion may result at points where the large particles or debris contact thesteel tank bottom. In this case, cathodic protection current will be shieldedand may not be effective in eliminating corrosion.

There are a wide variety of pad materials available, some of which may actually prevent the beneficial effects of cathodic protection. Conversely, there are situations where some of these materials, when properly selected and installed, can be beneficial in reducing corrosion to the extent that cathodic protection may not be needed, as described later in this section.


5.3.1.2 Soil resistivity may provide information about the corrosivity of the material used under and around a tank. A general resistivity classification isgiven in Table 1. There are several techniques for measuring soil resistivity. Acommon method is described in ASTM G57. It should be noted that soilresistivity alone should not be used to determine soil corrosivity.

5.3.1.3 The resistivity of the pad material may be higher than the existing surrounding soil. Corrosive soil beneath the higher resistivity pad material may contaminate the pad fill by capillary action and should be a consideration when determining sand pad thickness. Thus, resistivity of surrounding soil may be used to help determine the probability of corrosion on the tank bottom. The results of soil resistivity surveys should be considered to help determine the need for cathodic protection. However, other properties of the soil [see 5.3.2.1 g), h), i), and j)] should also be considered.


Table 1 - General Classification of Resistivity


5.3.1.4 In coastal areas, salt spray on tank surfaces will be washed down the sides of the tank by rain and may flow beneath the tank to contaminate thetank pad. This also can occur in areas where fertilizers or chemicals may bein the atmosphere either from spraying or industrial operations. The tank padalso can become contaminated by wicking action that can draw contaminatessuch as chlorides up from the water table. Cathodic protection is usuallynecessary for corrosion prevention in these situations.


5.3.1.5 If a leak occurs in a tank bottom, the leaking material can alsoinfluence corrosion on the external side. If water leaks from the tank, theenvironment under the tank may become more corrosive. If product leaksfrom the tank, it could create corrosion cells that did not previously exist oradversely affect the effectiveness of cathodic protection. A leak may washaway part of the pad material and eliminate the contact of the tank bottomwith the ground in some areas. Cathodic protection will not be effective insuch areas. Additionally, the drainage properties of the pad material may bedeteriorated by a leak and allow water and contaminates to remain in contactwith the tank bottom. The condition of the tank foundation should beevaluated after a leak to access change in corrosion properties of thefoundation. Leaked asphalt (for example) under a tank may partially shieldcathodic protection current from reaching the tank bottom.


5.3.2 Sand PadClean sand is the most common material used as a pad beneath aboveground storage tank bottoms. The use of clean sand alone normallydoes not eliminate the need for cathodic protection since corrosion may occurdue to intrusion of water from rain, snow, or a shallow water table,condensation, or other corrosion described in Section 4. Guidelines for thechemical and electrical analysis of the sand for corrosive contaminants arediscussed in 5.3.2.1 f), g), h), i), and j).

These are suggested guidelines or considerations for the construction of aboveground storage tank bottom sand pad (non-oiled) that were developed from practices represented by industry experience.


5.3.2.1 Sand Pad MaterialThe following are issues to sand pad material.a) The sand should be clean, screened, and debris free (i.e. no wood, sticks,

vegetation, paper, rocks; clay, silt, or other soil; welding rods or other metallic or non-metallic objects; etc.).

b) Cleaning should be done at the supply source and is accomplished by mechanical washing with water that will not alter the chemical composition or electrical resistance of the sand material.

Caution—A municipal potable water supply may not be acceptable because of chlorination which may produce high chloride levels.


c) Masonry mortar quality sand is sometimes specified.d) Sand should conform to specifications such as one of the following:

1) ASTM C778 type “20-30 sand” or equivalent, 2) ASTM C778 “graded sand” or equivalent, or 3) ASTM C144 or equivalent.

e) Portland cement, at an approximate 33:1 sand to cement ratio, or lime, at an approximate 95:5 (20:1) sand to lime ratio, is sometimes added at the supply source to elevate pH levels and/or to facilitate good compaction.

Caution—Do not use an excessive amount of Portland cement or lime as the pad material should be somewhat yielding and not hard like concrete in order to allow the tank bottom to uniformly bear on the sandpad material when loaded and make intimate contact with it.

Keywords:■ to elevate pH levels and/or ■ to facilitate good compaction


f) Electrical resistivity of the sand material is a commonly used method for determining its corrosivity because it is relatively easy to measure. Theresistivity of a soil depends on its chemical properties, moisture content, and temperature. The resistivity of the sand material may be determined inaccordance with ASTM G57, or equivalent. The results of the testing shall be forwarded to the cathodic protection designer.

g) Measuring pH indicates the hydrogen ion content of a soil. Corrosion of steel is fairly independent of pH when it is in the range of 5.0 to 8.0. The rate of corrosion increases appreciably when pH is < 5.0 and decreases when pH is > 8.0. pH may be determined in accordance with ASTM G51 or equivalent.

Keywords: Corrosion of steel is fairly independent of pH when it is in the range of 5.0

to 8.0. pH The rate of corrosion increases appreciably when pH is < 5.0 pH The rate of corrosion decreases when pH is > 8.0. pH


h) Chlorides will affect the resistivity of soil, and act as a depolarizing agent which will increase the current requirement for cathodic protection of steel.Pitting corrosion on steel can begin at chloride levels of 10 ppm. Chloridecontent may be determined in accordance with ASTM D512 or equivalent.There is currently no industry consensus on an acceptable range for chloride levels, therefore the tank owner/operator should specify theacceptable chloride level. There are practical and possible economic limitations in achieving minimum levels of chloride content.

i) Sulfate levels > 200 ppm frequently indicate high concentrations of organic matter. Sulfate content may be determined in accordance with ASTM D516 or equivalent. There is currently no industry consensus on anacceptable range for sulfate levels, therefore the tank owner/operator should specify the acceptable sulfate level. There are practical and possible economic limitations in achieving minimum levels of sulfate content.


j) Sulfide levels > 0.10 ppm, may indicate that sulfates have been reduced by bacteria. Sulfide content may be determined in accordance with EPA 0376.1 or equivalent. There is currently no industry consensus on anacceptable range for sulfide levels, therefore the tank owner/operator should specify the acceptable sulfide level. There are practical and possible economic limitations in achieving minimum levels of sulfide content.

k) Random testing of the sand material should be conducted at the supply source to determine if the electrical resistivity and chemical properties are at acceptable levels. Sand samples used to determine the properties of the material should be taken from the actual material that is to be used during construction. If test results do not meet owner/operator specified levels then additional steps such as rewashing and/or adding Portland cement or lime, or secure another source of sand, may need to be taken.


5.3.2.2 Sand Pad ConstructionThe following are considerations when constructing tank pads.a) Care should be taken to use clean mixing, handling, and construction

equipment to ensure that the sand pad material remains free from foreign matter and debris.

b) Sand is often installed by loose lifts in maximum layering of 6 in. to 8 in. (15 cm to 20 cm) thickness. The total depth or thickness of the sand pad is determined by engineering design and tank loading requirements and maybe from one to several layers thick dependent upon owner/operator specifications.

c) Mechanical vibratory compaction and rolling is suggested to be done for each layer to achieve compaction to 95% of the maximum dry density per ASTM D1557 or equivalent.

Caution—Compaction by water flooding is not recommended because the water used to flood the sand pad may cause contamination anddeterioration of the original chemical and electrical properties of the sand material. Additionally, flooding can cause bulking of the pad and subsequent adverse settlement when loaded, possibly causing damage to the tank bottom, attached piping or other appurtenances.


d) The bottom side of each steel plate to be used for tank bottom construction should be inspected immediately before placement onto the pad to ensurethat any contaminating debris that is adhered to it (e.g. mud) is removedand that the plate surface is clean.

e) Cathodic protection and leak detection materials/components may be placed in, or pass through, the sand pad only in accordance with owner/operator approved designs. Otherwise, the pad shall be completely free of any material other than the specified sand material.


5.3.3 Continuous Concrete Pad5.3.3.1 A properly designed concrete tank pad constructed on stable, properly prepared subsoil may be effective in eliminating intrusion of groundwater,soil-side corrosion, and the need for cathodic protection. Preparation of astable soil to support the concrete slab is very important to ensure thecontinued integrity of the pad. Unstable soil may induce cracks in the slabthrough which water and contaminants can permeate to the steel tank bottomand provide a corrosive environment.


5.3.3.2 Although corrosion from the soil may be prevented by a concrete pad, there may still be a collection of moisture between the tank bottom and thepad due to condensation, blowing rain or snow, flooding due to inadequatedrainage, rain water from the roof of the tank flowing down the tank walls, forexample. Corrosion may occur due to this moisture accumulation. Cathodicprotection is generally not considered an effective way to combat thiscorrosion. A free-draining concrete pad or ringwall and a seal around theperiphery of the tank may be effective in eliminating the accumulation ofmoisture between the pad and the tank bottom where flooding in the dikearea above the tank bottom does not occur. In situations where water maycondense on the tank bottom or water is retained above the concrete pad,accelerated corrosion may occur.


5.3.3.3 Due to numerous complex factors that can affect the corrosion of a tank bottom underside in the presence of concrete, prediction of thepropensity of corrosion in this case is extremely difficult. Thus, care should beobserved with tanks on concrete pads since cathodic protection may beineffective in this case. Consideration should be given to keep the surface ofthe concrete tank pad and steel bottom plate free of contaminants duringconstruction.


Continuous Concrete Pad


Corrosion Of A Tank Bottom


5.3.4 Crushed Limestone or Clam Shell PadIn certain locations, the tank pad could consist of a layer of crushed limestone or clam shells. Such tank pads without the use of cathodic protection haveproduced mixed results. The tank pad should be fine and uniform, sincedifferential aeration corrosion cells will cause pitting at contact areas betweenthe large particles and the metal. The intrusion of water from rain orgroundwater makes the environment under the tank alkaline, which mayreduce corrosion. If contaminants are present in the pad, or with time infiltratethe pad, corrosion may accelerate. Thus, the use of crushed limestone orclam shells does not clearly eliminate the need for cathodic protection.

Keywords:crushed limestoneclam shells


Clam Shell


5.3.5 Oiled Sand PadHistorically, in some cases oil has been added to the sand for various reasons, including compaction and corrosion control. However, if cathodicprotection is applied, the higher resistivity of oiled sand may prevent it frombeing effective. For new tank construction, use of oiled sand is discouraged.


Oiled Sand- Alberta

http://www.kqed.org/news/story/2013/01/08/114124/deep_in_canadian_lakes_signs_of_tar_sands_pollution


5.3.6 Continuous Asphalt PadA pad of new asphalt may provide many of the same advantages anddisadvantages as a concrete pad for reducing corrosion and eliminating theneed for cathodic protection. Proper support to prevent cracks and to preventaccumulation of water between the pad and the tank bottom is an importantconsideration. Asphalt degrades with time and can provide a path for waterand dissolved contaminants to come into contact with the steel tank bottom,allowing corrosion to occur. Cathodic protection, if applied, may or may notaid in stopping corrosion when the asphalt becomes deteriorated. In fact,deteriorated asphalt may shield cathodic protection current in a mannersimilar to a disbonded coating on a pipeline. The condition of the externalsurface of the tank bottom as well as the asphalt can be determined ifcoupons are cut from the tank bottom. For new tank construction, use ofasphalt pads is discouraged.


Continuous Asphalt Pad


5.3.7 Native Soil Pad5.3.7.1 Soil analysis is often a useful test for helping to determine whether the potential corrosion activity will be high enough to make cathodic protectionnecessary and whether cathodic protection will be a practical application toprevent corrosion. Determination of aggressive ions such as chlorides andsulfates along with measurement of pH and resistivity are helpful for furthercorrosion analysis. The variety of particle sizes and chemical and electricaldifferences as discussed in 5.3.1.1and 5.3.2.1 should also be considered inthe effectiveness of a cathodic protection system.


AST Tank Pad


5.4 Other Factors Affecting Cathodic Protection5.4.1 Contents of TankAboveground storage tank temperature can influence corrosion on tank bottoms. Accelerated corrosion can occur on the external surface of thebottom of heated tanks due to elevated temperatures if the area is wet.

NOTE :The corrosion rate of steel may double with every 18 °F (10 °C) increase intemperature above 77 °F (25 °C).

Aboveground storage tanks containing products above ambient temperature may require an increase in cathodic protection design current density to achieve adequate protection on the external surface of the bottom. Conversely, sufficient heat could dry out a well-drained tank pad, thus, increasing its resistivity and reducing performance of cathodic protection.


However, tank operators should be aware that if water penetrates the previously dried out tank pad (such as: flooding, condensation, blowing rain or snow, poor drainage, rooftop water), the resistivity of the tank pad can decrease, developing a more corrosive condition. For this situation, the installation of a cathodic protection system should be installed.


5.4.2 Bottom ReplacementReplacement of tank bottoms is an accepted practice. Cathodic protection systems can be used to ensure long-term integrity of existing andreplacement bottoms. The methods of installing replacement bottoms, releaseprevention barriers, and internal linings shall be considered when determiningthe need for and the method of installation of a cathodic protection system.These factors relate to both existing and new cathodic protection systems andhave a significant impact on the feasibility and effectiveness of cathodicprotection. The effect of replacement bottoms and release prevention barrierson cathodic protection system design is discussed in 7.2.


5.4.3 Release Prevention Barriers5.4.3.1 There are a variety of methods available for release prevention. These include, but are not limited to:a) use of an impervious conductive clay pad in tank dike;b) dual bottom tank design;c) impervious nonmetallic, nonconductive membrane;d) conductive and non-conductive liners or RPBs.

5.4.3.2 The use of release prevention barriers (RPB) will reduce the environmental risk in the event of a leak. However, the use of certain RPBs may preclude the use of cathodic protection and in some cases may cause accelerated corrosion of the tank bottom. An example of a double bottom release prevention system is the installation of a new steel bottom over an existing steel bottom which has been repaired. If water or other electrolyte intrudes into the interstitial space, a galvanic cell may be formed which will cause the new steel tank bottom to corrode at an accelerated rate. In order to apply cathodic protection to a new tank bottom, anodes shall be installed between the old and new bottom.


5.4.3.3 If a release prevention system utilizing a nonconductive, impervious external liner is in place or is installed under a new tank, the cathodicprotection anodes shall be placed between the external liner and tank bottomin a conductive material such as sand. Cathodic protection systems arerendered ineffective when an external liner is installed between the anodesand the tank bottom to be protected, because the external liner acts as ashield to the flow of cathodic protection current necessary for protection.

Another consequence of release prevention involves the use of an imperviousexternal liner which may trap corrosive liquid, resulting in more severecorrosion of the tank bottom. There are advantages and disadvantages tousing release prevention barriers and these should be evaluated to determinethe appropriate corrosion protection methodology. Refer to 7.2 for a furtherdiscussion of the effect of RPBs on cathodic protection design.


5.4.4 Thick-film Internal LiningsThe purpose of various types of internal tank linings is to mitigate internal corrosion threats and should not be considered as sufficient justification toeliminate the need for external cathodic protection of aboveground storagetank bottoms.


6 Methods of Cathodic Protection for Corrosion Control6.1 IntroductionCathodic protection is a widely accepted method of corrosion control. Corrosion of aboveground steel storage tank bottoms may be reduced oreliminated with proper application of cathodic protection. Cathodic protectionis a technique for preventing corrosion by making the entire surface of themetal to be protected act as the cathode of an electrochemical cell. There aretwo systems of cathodic protection – (1) galvanic and (2) impressed current.


6.2 Galvanic Systems6.2.1 GeneralGalvanic systems use a metal more active than the structure to be protected to supply the current required to mitigate corrosion (see Table 2 for a partialgalvanic series). The more active metal is called an anode, commonlyreferred to as a “galvanic anode.” The anode is electrically connected to thestructure to be protected and buried in the soil. A galvanic corrosion celldevelops and the active metal anode corrodes (is sacrificed) while the metalstructure (cathode) is protected (see Figure 5). Metals commonly used asgalvanic anodes in soil are (1) magnesium and (2) zinc in either cast or ribbon form. The anodes are usually distributed around the perimeter of the tank orpreferably buried beneath the tank bottom. Galvanic systems are normallyapplied only to small diameter tanks [e.g. less than 20 ft (7 m)] or for tankswith externally coated bottoms. Refer to 7.3.5.1 for a discussion on the designof galvanic cathodic protection systems.


Table 2 - Partial Galvanic Series


Figure 5 - Cathodic Protection with Galvanic Anodes


6.2.2 Advantages of Galvanic SystemsThere are several advantages of galvanic systems:a) no external power supply is required;b) installation is relatively easy;c) capital investment is low for small diameter tanks;d) maintenance costs are minimal;e) interference problems (stray currents) are rare;f) less frequent monitoring is required.

6.2.3 Disadvantages of Galvanic SystemsThere are several disadvantages of galvanic systems:a) driving potential is limited;b) current output is low;c) use is limited to low-resistivity soils;d) not practical for protection of large bare structures;e) very short life expectancy in low-resistivity soils.

Charlie Chong/ Fion Zhang http://pipelineprotection.co.uk/index.php/cathodic-protect


6.3 Impressed Current Systems6.3.1 GeneralThe second method of applying cathodic protection to an aboveground storage tank bottom is to use impressed current from an external source.Impressed current systems use direct current usually provided by a rectifierattached to an AC power source. The rectifier converts alternating current todirect current. Direct current from the rectifier flows to the buried impressedcurrent anode, from the anode through the soil electrolyte, and onto the tankbottom as shown in Figure 6. Refer to 7.3.5.2 for a discussion on design ofimpressed current cathodic protection systems.


Figure 6 - Impressed Current Cathodic Protection


6.3.2 Advantages of Impressed Current SystemsThe advantages of impressed current systems include:a) availability of large driving potential;b) high current output capable of protecting large or small structures;c) capability of variable current output;d) applicability to almost any soil resistivity.

6.3.3 Disadvantages of Impressed Current SystemsThe disadvantages of impressed current systems include:a) possible interference problems (stray currents) on foreign structures;b) loss of ac power causes loss of protection;c) higher maintenance and operating costs;d) higher capital cost for small installations;e) safety aspects of rectifier location;f) safety aspects of negative lead connection;g) more frequent monitoring.


6.3.4 Cathodic Protection RectifiersA typical cathodic protection rectifier has two major components: (a) a step-down transformer to reduce the AC supply voltage, and (b) rectifyingelements to convert AC input to DC output. Units may be obtained with eitherselenium or silicon rectifier elements. Silicon rectifiers are generally moreefficient; however, they are more susceptible to damage from power surges.Therefore, protective devices should be considered for these units to preventlightning damage.

Because of decreased life at higher temperatures, selenium rectifiers are notrecommended if ambient temperatures are expected to exceed 130 °F (55 °C).


6.3.5 Impressed Current AnodesImpressed current anodes used in soil are made of materials such as graphite, scrap steel, high silicon cast iron, or mixed metal oxides on titanium. Anodesare usually buried in a coke breeze backfill to extend their life and reducecircuit resistance. They may be located in remote anode beds, distributedaround the tank, installed underneath the tank, or installed in deep anodebeds. (>15m)


7 Design of Cathodic Protection Systems7.1 Introduction7.1.1 Cathodic protection systems are designed and installed to prevent corrosion of a tank bottom by satisfying the requirements of one or more of the criteria listed in Section 8. In order to achieve the desired results, a cathodic protection system shall be properly designed. The cathodic protection system should be designed after a study of the following items:a) design and engineering specifications and practices (see 5.2);b) operating procedures;c) safety, environmental, and hazardous area requirements;d) field testing.


7.1.2 In general, the design should provide adequate corrosion protection while minimizing installation, maintenance, and operation costs. The major objectives of cathodic protection designs for tank bottoms are to:

a) Deliver and distribute sufficient current to the external tank bottom to ensure that the criterion for protection is met.

b) Provide a design life of the anode system and other equipment commensurate with the design life of the tank or provide for periodic replacement of anodes and maintenance of equipment.

c) Provide adequate allowance for anticipated changes in current requirements with time.

d) Place anodes, cables, rectifiers, and test stations where the possibility of physical damage is minimal.

e) Minimize interference currents on neighboring structures.f) Provide sufficient monitoring points so measurements can be taken to

determine that the protection criterion is met on the entire surface of the tank bottom.


7.1.3 There are many factors to consider in the design of both internal and external cathodic protection systems. Cathodic protection systems should be designed only by a person thoroughly familiar with cathodic protectionpractices on aboveground storage tanks.

7.1.4 Whenever possible, the design should be based on standard components provided by manufacturers regularly engaged in the production of cathodic protection system components for aboveground storage tanks.


Standard Components Provided By Manufacturers Regularly Engaged In The Production Of Cathodic Protection System Components


7.2 Influence of Replacement Bottoms, External Liners (Release Prevention Barriers), and Secondary Containment on Cathodic Protection System Design7.2.1 Barriers to Cathodic ProtectionCathodic protection is achieved by directing the flow of current from an anode to a cathode, resulting in protection of the cathode. Anything that acts as a barrier or shield to the flow of current will prevent the application of cathodicprotection. RPBs and replacement bottoms can have such an effect if not properly considered.

7.2.2 Tank Bottom ReplacementReplacement of tank bottoms is an accepted practice. Whether the old bottom is left in place or removed has a significant impact on the types of cathodic protection systems that are feasible for corrosion control of the new bottom(see 7.2.4.3).


7.2.3 Considerations when External Liners are Used in a Diked Area7.2.3.1 Impervious External LinerOne method used to provide secondary containment is to line the entire diked area with an impervious external liner. An existing external liner under a tankor one proposed for a new tank can have a significant impact on the choicesand design of a cathodic protection system. In either case, if a nonconductiveliner is used, anodes shall be placed between the external liner and the tankbottom in order for cathodic protection to work (see Figure 7).

NOTE If a clay liner is installed instead of a nonconductive external liner, experience has shown that it will not significantly affect the operation of a conventional cathodic protection system.


Figure 7 - Impervious External Liner Beneath Aboveground Storage Tank


泥人 Clay Man – 关帝爷clay liner is installed instead of a nonconductive external liner, experience has shown that it will not significantly affect the operation of a conventional cathodic protection system.


7.2.3.2 Existing Multiple or Single Tanks With a Dike LinerTo install a cathodic protection system on an existing tank in a diked area that is lined from dike wall to dike wall and under the tank with an impervious non-onductive external liner, one option is to bore under the tank at a very shallowangle and install anodes under the tank. In order to obtain adequateprotection, an impressed current system probably would be required. Thiswould be feasible only if there was sufficient depth between the tank bottomand the external liner so that the integrity of the external liner would not becompromised. At this time, there is limited experience with this type of system.


7.2.3.3 New Multiple or Single Tanks with a Dike LinerNew tanks in a diked area lined with an impervious external liner can be equipped with a network of shallow anodes, a grid of ribbon or looped wiretype anodes which would be placed between the external liner and the tankbottom during construction. Such an arrangement could be an impressedcurrent system or a galvanic system. Alternatively, a geosynthetic clay linermay be used and a more conventional cathodic protection system installed.


TankJacking- Retrofitting tank bottom

http://www.quali-tek.com/?page_id=21


Geosynthetic Clay LinerA geosynthetic clay liner may be used and a more conventional cathodic protection system installed.


A geosynthetic clay liner (GCL) is a woven fabric-like material, primarily used for the lining of landfills. It is a kind of geomembrane and geosynthetic, which incorporates a bentonite or other clay, which has a very low hydraulic conductivity. The resulting lower permeability slows the rate of seepage out of the landfill. Due to environmental laws, any seepage from landfills must be collected and properly disposed of, otherwise contamination of the surrounding ground water could cause major environmental and/or ecological problems. The lower the hydraulic conductivity the more effective the GCL will be at retaining seepage inside of the landfill. Bentonitecomposed predominantly (>70%) of montmorillonite or other expansive clays, are preferred and most commonly used in GCLs. A general GCL construction would consist of two layers of geosynthetics stitched together enclosing a layer of processed sodium bentonite. Typically, woven and/or non-woven textile geosynthetics are used, however polyethylene or geomembrane layers or geogrid geotextiles materials have also been incorporated into the design or in place of a textile layer to increase strength. GCLs are produced by several large companies in North America, Europe, and Asia. The United States Environmental Protection Agency currently regulates landfill construction and design in the US through several legislations.

https://en.wikipedia.org/wiki/Geosynthetic_clay_liner


7.2.4 Replacement or Repair of Steel Tank Bottoms7.2.4.1 Steel tank bottoms may be repaired or replaced. Linings are frequently installed in tanks to protect against internal corrosion. An acceptable and commonly used method of replacing tank bottoms can be found in API RP 575 and API Standard 653.

7.2.4.2 The method of repair or bottom replacement is of major importance in determining if cathodic protection should be installed and whether or not an effective cathodic protection system can be installed.


7.2.4.3 If an existing tank bottom is protected by cathodic protection and/or if cathodic protection is planned for the new bottom (by means of deep orshallow anode beds), the old bottom will have to be completely removed. Asshown in Figure 8, if it is not removed, the old bottom forms a shield thatcollects the cathodic current flowing through the ground and preventscathodic protection of the new bottom. Unless the anodes are installedbetween the two steel bottoms (see Figure 8), or the old bottom is removed,electrically isolated, or coated with a nonconductive material, a galvanic cellcan develop between the old and new bottom. Industry experience hasshown that if a conductive electrolyte exists between bottoms, the current flowand metal loss will be from the new bottom, resulting in premature failure ofthe new bottom.


Figure 8 - New Steel Bottom on Top of Old Bottom


Industry experience has also shown that the successful operation and monitoring of some impressed anode systems installed in the interstitial space between the old and new tank bottom has been mixed when the distance between the two floors is less than 12 in. (30 cm). The closer thedistance between the two floors the greater the likelihood that the anodesystem and/or reference cells will become “shorted” to the new tank bottom atsome point in time due to normal loading and working of the tank. Regardlessof the distance between tank bottoms, adequate care should be taken toavoid shorting the anodes to the tank bottom.


The closer the distance between the two floors the greater the likelihood that the anode system and/or reference cells will become “shorted” to the new tank bottom at some point in time due to normal loading and working of the tank. Regardless of the distance between tank bottoms, adequate care should be taken to avoid shorting the anodes to the tank bottom.

distance between the two floors

“shorted”

Limited current distribution

Designed current distribution


7.2.5 Effects of Impervious Nonconductive External Liners7.2.5.1 GeneralThe installation of an external liner between the old and new bottom is an alternative practice. There are advantages and disadvantages to this practice.7.2.5.2 AdvantagesThere are several advantages to the installation of an external liner.a) An external liner provides a means of detecting and containing leaks and

preventing ground contamination if leaks occur.b) An external liner eliminates the natural current flow between the old

bottom and the new bottom, thus, reducing the accelerated failure of the new bottom due to galvanic corrosion.

c) An external liner may reduce entry of groundwater into the space between the bottoms.


7.2.5.3 DisadvantagesInstalling an external liner could be disadvantageous for the following reasons.a) An external liner makes the future addition of cathodic protection difficult.(?)b) An external liner acts as a basin to contain water or any other electrolyte

that might wet the sand between the old and new bottoms, thus, increasing corrosion rates.


7.2.5.4 Mitigation of Adverse EffectsTo keep the advantages and eliminate or reduce the adverse effects of a external liner in the old bottom, install a cathodic protection system in thespace between the old bottom and the new bottom. Such a system could consist of an array of ribbon or wire anodes embedded in the sand betweenthe old bottom and the new bottom. The connecting wires project through theexternal liner and through a sealed bulkhead fitting (in the old shell portion) tobe connected directly to the tank or through a test station. This type of systemwould have to be installed at the time the bottom is replaced. If unexpectedanode depletion or failure occurs, remedial measures to replace anodescannot be easily accomplished.


The advantages of this system include the following:a) the need for future cathodic protection system installation is decreased;b) the external liner acts as a barrier to current flow, directing the current flow

toward the new bottom;c) as long as the sand stays dry and remains free of chemical contaminants,

corrosion rates and current flow will be low as a result of the high resistivity of the sand;

d) corrosion would tend to increase if the sand became wet; however, the resistivity of the sand would be much lower, thus, more current would flow and cathodic protection would increase (a self-governing system);

e) the cost of the anodes is a small fraction of the total cost.


7.3 External Cathodic Protection7.3.1 IntroductionThe purpose of this section is to recommend procedures for designing cathodic protection that will effectively control external corrosion by satisfying one or more of the criteria listed in Section 8 for the intended service life of the aboveground storage tanks.

7.3.2 Design ConsiderationsIn the design of cathodic protection, the following items should be considered.a) Recognition of hazardous conditions existing at the proposed installation

site and the selection and specification of materials and installation practices that will ensure the safe installation and operation of the cathodic protection system.

b) Specification of materials and installation practices to conform withapplicable codes, such as National Electrical Manufacturers Associationstandards, NACE recommended practices, and federal, state, and localregulations.


c) Selection and design of the cathodic protection system for optimum economy of installation, maintenance, and operation.

d) Selection and specification of materials and installation practices that will ensure dependable operation throughout the intended service life of the cathodic protection system.

e) Selection of a design to minimize excessive protective currents or earth potential gradients that can detrimentally affect the tanks, piping, coating, or neighboring buried or submerged metallic structures.

f) Provisions for monitoring the cathodic protection system operation.


7.3.3 Information Useful for DesignInformation that is useful for design can be divided into three categories:a) specifications and practices;b) site conditions;c) field survey, corrosion test data, and operating experience.


7.3.3.1 Specifications and PracticesInformation pertaining to these specifications or practices may prove useful:a) site plan and system layout;b) construction dates;c) tank design information; coned up bottom, coned down bottom, “w” bottom,

flat bottom;d) tank foundation design: earthen based, depth of ringwall, pilings, concrete

cap/slab under tank, etc.;e) pumps and power supply;f) coatings;g) corrosion control test stations;h) electrical isolation;i) electrical bonds;j) electrical conduit routing;k) electrical area classification boundaries.


Electrical Area Classification Boundaries- API RP500


7.3.3.2 Site ConditionsThese factors relating to site conditions are considerations in designing

cathodic protection:a) existing and proposed cathodic protection systems;b) possible interference sources (stray current);c) special environmental conditions;d) depth of bedrock;e) depth of frost line;f) neighboring buried metallic structures (including location, ownership, and

corrosion control practices);g) structure accessibility;h) power availability;i) feasibility of electrical isolation from foreign structures;j) release prevention barriers and secondary containment systems, if any;k) areas of poor water drainage;l) depth of water table;m) depth of soil strata.


7.3.3.3 Field Survey, Corrosion Test Data, and Operating ExperienceThe following information may also be useful:a) protective current requirements to meet applicable criteria;b) electrical resistivity of the electrolyte (soil);c) electrical continuity of the tanks and connected piping;d) electrical isolation of structures;e) coating integrity;f) leak history of similar structures in the area;g) deviation from construction specifications;h) existence of stray current;i) other maintenance and operating data.


7.3.4 Considerations that Influence Selection of the Type of Cathodic Protection System (impressed or sacrificial)

7.3.4.1 The following factors influence the selection of a cathodic protection system:

a) size and number of tanks to be protected;b) current required;c) soil conditions such as resistivity, chemical composition, aeration, and pH;d) possibility of cathodic protection interference on adjacent structures;e) future developments and extensions to the storage system;f) cost of cathodic protection equipment, installation, operation, and

maintenance;g) existing or proposed release prevention barrier and/or secondary

containment system;h) the type of liner specified;i) anode separation from the tank bottom.


7.3.4.2 Several options are available for the protection of one or more tanks, including:

a) shallow anodes installed around the periphery of the tank;b) anodes installed directly under the tank prior to construction;c) boring at an angle or horizontally directional drilling under the tank so that

anodes can be installed beneath it in a pattern that will provide adequate protection;

d) use of a deep anode bed.


Shallow Bed Anodes


Sacrificial Zn Anode with Backfill


7.3.5 Types of Cathodic Protection Systems7.3.5.1 Galvanic Anode System7.3.5.1.1 Galvanic systems use galvanic anodes which can be made of materials such as magnesium or zinc in either cast or ribbon form. These areinstalled either bare or packaged in a special backfill. The anodes areconnected to the system, either singly or in groups. Galvanic anodes arelimited in current output by the system-to-anode driving voltage and the circuitresistance. It may be more economical to cathodically protect bare, poorlycoated, or large structures with impressed current-type systems. Galvaniccathodic protection systems may be more economical on small diametertanks [less than 20 ft (6 m); see NACE RP0193 and 6.2 of this RP].

7.3.5.1.2 Three galvanic anode materials are commonly used for soil installations:a) high potential magnesium alloys; Mgb) standard magnesium alloy; Mgc) zinc. Zn

Note: No Aluminum!


7.3.5.1.3 Magnesium and zinc anodes prepackaged in special backfill are readily available in a number of size and weight configurations to meetvarious current output and anode life design requirements. The use of a special back fill with anodes is desirable for installation in soil environments.Special backfill, consisting of a proper mixture of gypsum, bentonite, andsodium sulfate, promotes anode efficiency, lengthens anode life, and keepsthe anode environment moist.

7.3.5.1.4 The number of anodes required to provide cathodic protection for aboveground storage tanks depends upon total current requirements and the expected individual anode discharge rate in the soil. In placing the anodes,current distribution factors should also be considered. Sometimes it is advantageous to consider the use of different sizes of anodes so that more anodes will be required and give better current distribution. Typically, better current distribution and uniform polarization is obtained by distributing anodes around the tank or under the tank (for new construction).


7.3.5.2 Impressed Current System7.3.5.2.1 Anodes7.3.5.2.1.1 Impressed current anodes can be of materials such as, but not limited to, graphite, high silicon cast iron, scrap steel, platinized metals, magnetite, and mixed metal oxides. These anodes are installed either bare or in special backfill material (usually coke breeze). They are connected with an insulated conductor either singly or in groups to the positive terminal of a direct current source. The structure is connected to the negative terminal of the direct current source.

7.3.5.2.1.2 Graphite, high silicon cast iron, or mixed metal oxide anodes are generally preferred for soil installations. Platinized niobium, tantalum, and titanium are best suited for water rather than soil installations.


7.3.5.2.1.2 Graphite C, high silicon cast iron Si-Fe , or mixed metal oxide MOx anodes are generally preferred for soil installations.


Platinized niobium Nb , tantalum Ta , and titanium Ti are best suited for water rather than soil installations.


7.3.5.2.1.3 Each anode material has an optimum current density that providesmaximum anode service life. Anodes may be located in remote anode beds, located in deep anode beds, or distributed closely about or under the structure to be protected. A proper anode bed design should:a) avoid physical interference with existing facilities;b) provide uniform current distribution; andc) avoid stray current interference with offsite structures.

7.3.5.2.1.4 The number of anodes in a particular cathodic protection design will be determined by total current requirements of the structures to be protected and the optimum current density of the anode material selected. For a distributed anode design, additional anodes may be installed to provide more uniform current distribution and to provide allowance in case of isolated anode connection failures or partial anode depletion.

Keywords:■ total current requirements■ optimum current density■ isolated anode connection failures or ■ partial anode depletion.


7.3.5.2.2 Current and Voltage Requirements7.3.5.2.2.1 For optimum design, the current required for cathodic protectionshould be calculated using the results of current requirement tests. However, in lieu of a current requirement test, the generally accepted protective current density is between 1 mA/ft2 and 2 mA/ft2 at ambient temperatures (0.5 mA/ft2for close coupled anodes), and current density between 2 mA/ft2 and 8 mA/ft2for elevated temperature tanks.

If a current requirement test is used, it can be performed only on existing tanks. It is conducted using a temporary anode bed and an appropriate source of direct current (see Figure 9). The temporary anode bed is typically positioned in the soil near the perimeter of the tank. Depending on the current required, the power source can vary from a 12-volt storage battery to a 300-amp welding unit.


at ambient temperaturescurrent density between 1 mA/ft2 and 2 mA/ft2 at ambient temperatures (0.5 mA/ft2 for close coupled anodes), and

for elevated temperaturecurrent density between 2 mA/ft2 and 8 mA/ft2 for elevated temperature tanks.


Figure 9 - Current Requirement Test Setup


7.3.5.2.2.2 Current requirement tests are conducted by forcing a known amount of current from the temporary anode bed through the soil and onto the tank to be protected. The degree of protection at various locations around the tank, and under the tank center if possible, is evaluated using potential measurements. This testing allows approximation of the current required to protect the tank. Current requirement tests should be conducted with an adequate liquid level in the tank to maximize contact of the tank bottom with the pad material.

7.3.5.2.2.3 The voltage necessary to drive the required amount of current depends largely on the number and location of anodes and on the resistivity of the soil. Since the current is generally known from a current requirementtest or is estimated, the voltage requirement can be calculated using Ohm’s Law (E = I × R) if the resistance of the circuit is known. The resistance can be estimated in several ways.


a) From existing impressed current systems similar to the one to beinstalled.

b) From current requirement tests as described above if the test anode bed is similar to the final one.

c) From soil resistivity tests, the anode-to-earth resistance can be calculated using a variation of Dwight’s equation

(see Peabody’s Control of Pipeline Corrosion).

The anode-to-earth resistance is generally the dominant part of the overall circuit resistance in an impressed current system.

http://www.corrosion-doctors.org/CP/soil-resist.htm


7.3.5.2.3 Rectifier SelectionThe rectifier output capacity selected will depend upon the following factors.a) Estimated or measured current requirements of the structure to be

protected.b) The voltage needed to cause current to flow from the anodes to the buried

structure.c) Rectifiers with a moderate excess capacity (typically 10 % to 50 %) should

be selected to allow for adjustments during the life of the cathodic protection system and to prevent damage due to voltage overloads. Care should be observed if the excess rectifier capacity will be used to drive the anodes. Increasing the current output of the anodes above the manufacturer’s rating will cause (1) a decrease in anode life and can (2) cause increased circuit resistance by drying out the backfill around the anodes.


7.3.6 Electrical Isolation7.3.6.1 Isolation devices consisting of flange assemblies, prefabricatedisolating joints, or couplings should be installed where electrical isolation ofthe system is required to facilitate the application of corrosion control. Thesedevices should be properly rated for temperature, pressure, and dielectricstrength. Isolating devices should not be installed in enclosed areas wherecombustible atmospheres are normally present.

7.3.6.2 Electrical grounding of electrical equipment is an essential element in personnel safety. It may not be practical or desirable to electrically isolate aboveground storage tanks from each other or adjacent plant equipment due to the probable interconnection via a grounding grid. Attachment of auxiliary equipment such as electronic gauges, mixer motors, and lighting may also preclude the possibility of effectively maintaining isolation. When installing a cathodic protection system for a tank bottom, it should be recognized that some of the current may also be collected on adjacent buried metallic equipment. De-coupling devices should be considered when isolating electrical grounding systems from aboveground storage tanks and/or piping.


7.3.6.3 Lightning arresters and fault current protection devices installed at electrical isolation locations should be of a size suitable for short term, highcurrent loading. Where isolation devices are installed in areas of known orsuspected influence from high voltage alternating current (HVAC) power lines,precautions should be taken to ensure that the AC potentials across suchdevices do not present a hazard to personnel. Refer to the latest revision ofNACE SP0177.

Note:■ NACE SP0177Mitigation of Alternating Current and Lightning Effects on Metallic Structures and Corrosion Control Systems


7.4 Internal Cathodic Protection7.4.1 The design of an internal cathodic protection system is complicated because of the variation in the level of the accumulated corrosive medium (usually water). In addition, the presence of sludge and other contaminants could have a detrimental effect on the performance of the cathodic protection system. There are many factors which influence the design of an internal cathodic protection system. A few of these factors are as follows:

a) condition and type of coating (if any);b) minimum and maximum water level in tank;c) compatibility of stored liquid with anodes and cables;d) internal inspection interval of tank, which affects design life;e) tank bottom shape: coned up, coned down, “w” type bottom, flat bottom.


7.4.2 Because of the many variables associated with design of internalcathodic protection systems for aboveground petroleum storage tanks, the use of these types of systems is limited and industry wide practices do not exist. It is recommended that Section 4 and Section 5 of NACE SP0575 be consulted when designing similar systems for aboveground hydrocarbon storage tank bottoms. In addition, NACE SP0388 should be consulted for design information for impressed current systems.

Note:■ NACE SP0575Internal Cathodic Protection (CP) Systems in Oil-Treating Vessels■ NACE SP0388Impressed Current Cathodic Protection of Internal Submerged Surfaces of Carbon Steel Water Storage Tanks


8 Criteria for Cathodic Protection8.1 IntroductionThe purpose of this section is to list criteria for cathodic protection that will indicate when adequate cathodic protection has been achieved. The selectionof a particular criterion for achieving the objective depends, in part, on priorexperience with similar structures and environments wherein the criterion hasbeen used successfully.

8.2 Protection Criteria8.2.1 There are several criteria used to determine if adequate cathodic protection has been achieved on aboveground storage tanks. For a more detailed description, refer to the latest edition of NACE RP0193.8.2.2 The following criteria were developed through laboratory experiments or were determined empirically by evaluating data obtained from successfully operated cathodic protection systems. It is not intended that persons responsible for corrosion control be limited to these criteria if it can be demonstrated that the control of corrosion can be achieved by other means.


8.2.2.1 A negative (cathodic) potential of at least 850 mV with the cathodic protection current applied. This potential shall be measured with respect to a saturated copper/copper sulfate reference electrode (CSE) contacting the electrolyte. Voltage drops other than those across the tank bottom-to-electrolyte boundary shall be considered for valid interpretation of this voltage measurement.

NOTE “Considered” is understood to mean the application of good engineering practice in determining the significance of voltage drops bymethods such as:

a) measuring or calculating the voltage drop(s),b) reviewing the historical performance of the cathodic protection system,c) evaluating the physical and electrical characteristics of the tank bottom

and its environment, andd) determining whether or not there is physical evidence of corrosion.


8.2.2.2 A negative polarized potential of at least 850 mV relative to a CSE. (One common method of measuring polarized potential is by using the “instant off” technique.) This criterion may be excessive and not practical for bare steel tank bottoms.8.2.2.3 A minimum of 100 mV of cathodic polarization measured between the tank bottom metallic surface and a stable reference electrode contacting the electrolyte. The formation or decay of this polarization can be measured tosatisfy this criterion.


8.3 Measurement Techniques8.3.1 The standard method of determining the effectiveness of cathodicprotection on a tank bottom is the tank-to-soil potential measurement. Thesemeasurements are performed using a high-impedance voltmeter (greater than10 M ohms) and a stable, reproducible reference electrode contacting theelectrolyte. These measurements are commonly made with the referenceelectrode placed in contact with the soil at several places around theperimeter of the tank as shown in Figure 10 and, if possible, at one or morepoints under the tank. Undertank measurements are made becausemeasurements at the perimeter of the tank may not represent the tank-to-soilpotential of the center of the tank bottom. Methods to monitor tank-to-soilpotentials under the center of the tank are discussed in 9.4. If undertankpotential measurements are not used, a good engineering practice, inconjunction with the inspection and maintenance practices of API 653, shouldbe used to determine that cathodic protection is adequately controllingcorrosion of the tank bottom.


Figure 10 - Potential Measurement Schematic


8.3.2 The actual structure-to-soil potential under the tank should be determined by using permanently installed reference electrodes, or bytemporarily inserting a waterproof, portable reference electrode under thetank through a perforated nonmetallic tube (see 9.4). With a perforated tube,it may be possible to determine the least protected area of the tank bottomand thereafter use that one point as the primary undertank monitoring location.Permanently installed reference electrodes may vary from their originalreference potential. This possible variation should be taken into account whenevaluating structure-to-electrolyte potentials.

8.3.3 The tank-to-soil potential measurements may be taken with current applied; however, consideration of IR drop(s) in the soil shall be made. Consideration of the IR drop in the soil is necessary for measurements made at the tank perimeter even if the reference electrode is placed immediately adjacent to the tank. This is especially true if distributed anodes are close to the tank since the perimeter of the tank may be within the electric field gradient of the anodes.


8.3.4 The value of the IR drop and the methods of consideration should be determined by using good engineering practices. Interrupting the flow ofcurrent at the rectifiers using the “instant-off” technique is a common method.The IR drop, once determined, can be used for future tests at the samelocation if conditions remain similar.8.3.5 Tank bottom surface area contacting the tank pad may vary with the tank content level. Since this condition can cause variations in tank-to-soil potentials, the level of the tank contents should be considered at the time of the survey. For more detailed information, see NACE TM0497.


8.4 Alternative Reference ElectrodesOther standard reference electrodes may be substituted for the saturated copper/copper sulfate reference electrode. Three commonly used referenceelectrodes are listed in Table 3, along with their voltage equivalent to –0.85volt referred to a saturated copper/copper sulfate reference electrode.

Table 3 - Commonly Used Reference Electrodes


NACE RP0193Standard Recommended Practice - External Cathodic Protection of On-Grade Metallic Storage Tank Bottoms

NACE TM0497Field Corrosion Evaluation Using Metallic Test Specimens


9 Installation of Cathodic Protection Systems9.1 IntroductionThe purpose of this section is to recommend procedures for the installation of cathodic protection systems that will control corrosion of the tank bottom ifdesign considerations recommended in Section 7 have been followed. Theinstallation of cathodic protection systems should be under the supervision oftrained and qualified personnel to ensure that the installation is made in strictaccordance with the drawings and specifications. Exceptions may be madeonly with the approval of the owner, operator, or personnel qualified by theowner or operator.


9.2 Galvanic Anode Systems9.2.1 Packaged anodes should be inspected to ensure integrity of the container and should be kept dry during storage. If individually packagedanodes are supplied in waterproof containers, that container shall beremoved before installation. Electrical continuity between the anode and leadwire should be tested without compromising the integrity of the package.Packaged galvanic anodes should be backfilled with compacted native soil.Figure 11 shows a typical galvanic anode installation.

9.2.2 Where anodes and special backfill are provided separately, anodes should be centered in the special backfill, which should be compacted prior to backfilling with native soil.


Figure 11 - Typical Galvanic Anode Installation


9.2.3 When galvanic anodes are used to protect internal surfaces of tank bottoms, they can be either bolted or welded to the tank bottom and can beshaped into slabs or ribbons and distributed along the tank bottom. Theconnection should be coated, but care should be taken not to coat or paint theanode. The anodes must be placed in the electrolyte to perform as designed.

9.2.4 Where a ribbon-type anode is used between tank bottoms, it isgenerally installed in clean dry sand. Ribbons should be carefullystraightened and made to lie flat so that the anode will not contact the steelbottom. No backfill other than sand is installed around ribbon type anodesbetween tank bottoms.

9.2.5 Care should be taken so that lead wires and connections are not damaged during backfill operations. Lead wires should have enough slack to prevent strain. Anodes should not be carried or lowered into the excavation by the lead wire.


9.3 Impressed Current Systems9.3.1 General9.3.1.1 Impressed current anodes should be inspected for defects, conformance to anode material specification, size and length of lead wires,and to ensure that the anode cap, if used, is secure. Care shall be exercisedto avoid cracking or damaging anodes during handling and installation.Cracked anodes should not be used. Lead wires should be carefullyinspected for defects in insulation. Care shall be taken to avoid damage toinsulation on wire. Defects in the lead wire shall be repaired, or the anodeshall be rejected.


9.3.1.2 Impressed current anodes can be installed vertically or horizontally by conventional excavation or augering, angled by boring, in deep vertical holesby drilling, or horizontally by directional drilling. Impressed current anodes aretypically installed in carbonaceous backfill such as coke breeze. If the backfillis installed properly so that there are no voids around the anode, much of thecurrent reaching the anode is conducted to the backfill by electrical contact.This promotes consumption of the backfill instead of the anode andsubstantially lengthens the effective anode life. Carbonaceous backfill alsotends to reduce total circuit resistance by lowering anode-to-soil resistance.For new construction, installation of the anodes under the tanks should beconsidered a best practice.

Keywords:■ This promotes consumption of the backfill instead of the anode andsubstantially lengthens the effective anode life.■ Carbonaceous backfill also tends to reduce total circuit resistance by lowering anode-to-soil resistance.


9.3.1.3 The following are principal points to be observed in the installation of impressed current anodes.

a) The coke breeze shall be correctly installed because loose backfill can result in high resistance and shortened anode life. The anode should becentered in the coke breeze. Premature anode failure will occur if the anode comes in contact with the soil. Carbon backfill is consumed over time at a rate of 2.2 lb (1.0 kg) per amp year. Enough carbon backfill needs to be uniformly installed around the anodes for the anode to achieve its designed life expectancy.

b) Buried anode connections shall be protected with extreme precautions against the entrance of any moisture, because any discharge of current to earth from the cable will destroy it very rapidly.

c) Care should be taken to protect the cable connection to the anode; this is the weak point in all anodes, and the joint is prone to failure by the entrance of moisture through even the tiniest crack.

d) Anodes and cable should be installed at a sufficient depth to protect against accidental damage. The anode lead may be severed by corrosion if there is the slightest break in its insulation.


9.3.2 Shallow Anode Bed Installation9.3.2.1 Figure 12 shows an example of a shallow anode bed installation. For a typical vertical anode installation, the hole is excavated 8 in. to 12 in. (20 cmto 30 cm) in diameter by approximately 10 ft to 20 ft (304 cm to 366 cm ? ) deep. Power auger equipment is used where available if both the terrain and right of way will permit. The anode is centered in the opening and properly installed anode backfill is carefully tamped when necessary. Many anodes also come prepackaged in carbonaceous backfill.

9.3.2.2 Sometimes it is necessary to install an anode in a location where rock is encountered at a shallow depth, or where soil resistivity increases markedly with depth. Such sites can be coped with by a horizontal installation of anodes. A ditch is excavated to whatever depth is practical, and a horizontal column of coke breeze is laid therein, usually square in cross section. The anode is laid horizontally in the center of this column. Groundbed resistances tend to be higher for horizontally installed groundbeds. Additional anodes or increased rectifier voltage should be considered with horizontally installed groundbeds.


Figure 12 - Typical Shallow Anode Bed Installation


9.3.2.3 In some instances, to improve current distribution to the center of the tank, it may be desirable to install anodes in holes which are bored at an angle under the perimeter of the tank bottom or bored horizontally underneaththe tank bottom by directional drilling. Prepackaged anodes may be beneficial in such installations to ensure that the anode remains centered in the coke breeze column. Alternatively, centering a wire- or cable-type anode (e.g. Mixed metal oxide, etc.) with centralizers in a perforated nonmetallic tube prior to coke breeze placement can be beneficial in horizontal directionally drilled installations to ensure that the anode remains centered in the coke breeze column.


9.3.3 Deep Anode Bed InstallationIn situations where a deep anode bed similar to that shown in Figure 13 is required, refer to the latest edition of NACE SP0572. Prior to installation of adeep anode bed, it is important to consider the environmental aspects since the anode bed may be installed through underground aquifers. It is often appropriate to provide an internal and external casing seal to maintain separation between surface and subsurface environments.

Note:SP0572Standard Practice Design, Installation, Operation, and Maintenance of Impressed Current Deep Anode Beds


Figure 13 - Commonly Installed Deep Anode Bed


Figure 13 - Commonly Installed Deep Anode Bed

It is often appropriate to provide an internal and external casing seal to maintain separation betweensurface and subsurface environments.


9.3.4 Rectifier Installation9.3.4.1 The rectifier or other power source should be installed so that the possibility of damage or vandalism is minimized.

9.3.4.2 Rectifiers and associated wiring should comply with local, state, and national electrical codes and electrical area classification per API 500. Anexternal disconnect switch on AC wiring should be provided. The rectifier case should be properly grounded.

9.3.4.3 Lead wire connections to the rectifier shall be mechanically secure and electrically conductive. Before the power source is energized, it shall be verified that the negative lead is connected to the structure to be protected and that the positive lead is connected to the anodes.

Caution— If the leads are reversed, with the positive lead mistakenly attached to the tank, the tank bottom will serve as an anode and rapid corrosion failure can result.


9.3.4.4 An exothermic weld connection is the preferred means for connecting the negative rectifier lead wire to the structure to be protected; however, goodmechanical connections may be substituted if necessary. All positive cableconnections and wire splices should be carefully waterproofed and covered with electrical insulating material. If mechanical connections are used, they should not be buried.


9.3.5 Cable Installation9.3.5.1 All underground wire attached to the positive rectifier terminal is at a positive potential with respect to ground. If not completely insulated, the wire may discharge current (act as an anode), which will result in corrosion of the wire and rapid failure of the cathodic protection installation. Therefore, all anode lead wires, header cables, and any wire splices should be carefully inspected prior to backfilling. Cable can be installed using common excavation techniques. Proper precautions should be taken to prevent damage to buried structures. Backfill should be free of sharp stones and other material that could damage wire insulation. Consideration should be given to installing cable in rigid conduit in areas subject to frequent excavation or where cable insulation is prone to damage by rodents.

9.3.5.2 Underground splices of the header cable (positive lead wire) to the anode bed should be avoided. Connections between header cable and anode lead wires should be mechanically secure and electrically conductive.Sufficient slack should be left to avoid strain on all wires. All splices and connections shall be sealed to prevent moisture penetration so that electricalisolation from the environment is ensured.


Rodent


9.4 Corrosion Control Test Stations, Undertank Monitoring Methods, and Bonds9.4.1 The structure and test lead wires should be clean, and dry at points of connection. Connections of test lead wires to the structure shall be installed so that they will remain mechanically secure and electrically conductive. A preferred method from the electrical standpoint is the use of an exothermic weld connection. However, this method is not recommended in areas where a combustible atmosphere may exist during the attachment process.

9.4.2 Attention shall be given to the manner of installing test lead wires for corrosion control testing to avoid affecting the physical strength of the structure at the point of attachment.

9.4.3 All test lead wire should be coated with an electrically insulating material.Test lead wires should be color coded or otherwise permanently identified.Sufficient slack should be left to avoid strain on all wires. Damage to wireinsulation shall be avoided, and proper repairs shall be made if damage occurs.


9.4.4 One of the problems associated with monitoring cathodic protection systems on tank bottoms is the inability to place a portable referenceelectrode in close proximity to the underside. For new tank construction, one or more of the following should be installed to permit testing the tank-to-soil potential at one or more places under the tank bottom.

a) Permanently installed reference electrodes and lead wires underneath the tank pad to the perimeter of the tank where they can be terminated in a test station for future use in testing (see Figure 14). Industry experience has shown that reliability of reference electrodes has been questionable; installation of redundant electrodes should be considered. Table 4 can be used for guidance for reference electrode quantities.

b) Perforated nonmetallic tubes for use in measuring the tank-to-soil potential at locations along the length of the tube should be installed (see Figure 15).


Figure 14 - Permanently Installed Reference Electrode and Test Station


Figure 15 - Perforated Pipe Installed for Reference ElectrodeCAUTION Care shall be exercised when using directional boring techniques under aboveground storage tanks to prevent undermining of the tank pad and to avoid damaging the tank bottom.


Table 4 - Permanently Reference Electrodes


9.4.5 When a tank bottom is repaired or replaced, one or more permanently installed reference electrodes and/or perforated nonmetallic tubes should beinstalled under the tank bottom. For existing tanks not scheduled for suchrepairs, one or more perforated nonmetallic tubes should be installedunderneath the tank using a directional boring technique shown in Figure 15.Reference electrodes are inserted in the perforated pipe, which provideselectrical continuity between the soil outside the tube and the referenceelectrode inside the tube for the entire length. The reference electrode shouldbe inserted into the pipe, using a nonmetallic electrician’s fish tape or smalldiameter PVC pipe, to obtain a profile of the tank-to-soil potentialmeasurements across the tank bottom. If a metallic device is used to insertthe reference electrode, it should be removed before readings are taken.


9.4.6 Where permanently installed reference electrodes are specified, installation should be per manufacturer’s recommended procedures.9.4.7 It is common to contact the tank with a sharp point, such as an awl, when taking a tank-to-soil potential measurement. This repeated action can cause early failure of the tank’s paint system if not recoated. Permanentlyinstalled test leads, grounding lugs, or short pieces of cable or tubing can avoid this and also readily identify the normal monitoring locations.9.4.8 If isolating devices are required, inspection and electrical measurements should be made to ensure that electrical isolation is effective and meets the requirements for cathodic protection.


9.4.9 Electrical devices isolated from the liquid storage system for cathodic protection purposes should be provided with safety grounds in accordance with applicable electrical codes.9.4.10 Cathodic protection coupons placed under the tank bottom and used in conjunction with permanent reference electrodes is an effective method of determining polarized potentials and IR-drops.9.4.11 The installation of Electrical Resistance Probes (?) permanently inserted through the ring wall and under the tank bottom can be useful in determining the corrosion rate of the tank bottom and the efficiency of the cathodic protection system.


10 Interference Currents10.1 IntroductionThe purpose of this section is to identify sources of interference currents and to recommend practices for the detection and control of these currents. Itshould be noted that the installation of a new impressed current cathodicprotection system may cause interference with neighboring structures.Interference is the undesirable discharge of current from a structure causedby the application of electrical current from a foreign source. Interference isnormally from a DC source, although AC can also cause interferenceproblems. A more detailed description can be found in 4.2.1. Consult thelatest edition of NACE RP0193 for more information on interference currents.

Note:NACE RP0193Standard Recommended Practice - External Cathodic Protection of On-Grade Metallic Storage Tank Bottoms


10.2 Sources of Interference Currents10.2.1 Constant CurrentThese sources have essentially constant direct current output (DC) . The most common sources are rectifiers energizing nearby cathodic protection systems.10.2.2 Fluctuating CurrentThese sources have a fluctuating direct current output (DC) . The greatest sources of stray current are the electric railway, rapid transit system, underground mining electrical systems, and welding machines.


10.3 Detection of Interference CurrentsDuring corrosion control surveys, personnel should be alert for electrical or

physical observations that could indicate interference from a neighboring source. These include:

a) an electronegative shift of the historical structure-to-soil potential data on the affected structure at a point where current is picked up from the foreign direct current source;

b) an electropositive shift of the historical structure-to-soil potential data on the affected structure at a point where current may be discharged from the affected structure; And

c) localized pitting in areas near or immediately adjacent to a foreign structure.

Whenever a new grounbed is installed in the vicinity of other buried structures or output increased on an existing groundbed in the vicinity of other buried structures, interference testing should be performed.


an electronegative shift - picked up from the foreign direct current source; an electropositive shift - discharged from the affected structure;


10.4 Control of Interference Currents10.4.1 Multiple approaches may have to be employed to resolve an

interference problem:a) design that aims at minimizing exposure;b) managing current output of all sources to minimize interference currents;c) bonding to provide a metallic return of current collected from the interfered

with structure to the interfering structure;d) auxiliary drainage of the collected current by the use of galvanic anodes.

10.4.2 Interference problems can be prevented and resolved by participation in local corrosion coordinating committees. When interference effects are observed, the committee can often provide information on the source of the interference currents. If no local committee exists, refer to NACE TPC 11 and/or contact NACE International for information regarding these committees.


Interference Currents


11 Operation and Maintenance of Cathodic Protection Systems11.1 Introduction11.1.1 The purpose of this section is to recommend procedures and practices for energizing and maintaining continuous, effective, and efficient operation ofcathodic protection systems. Electrical measurements and inspections arenecessary to determine that protection has been established according toapplicable criteria and that each part of the cathodic protection system isoperating properly. Conditions that affect protection are subject to changewith time. Corresponding changes may be required in the cathodic protectionsystem to maintain protection. Periodic measurements and inspection arenecessary to detect changes in the cathodic protection system. Conditionsmay exist where operating experience indicates that testing and inspectionsshould be made more frequently than recommended herein.


11.1.2 Care should be exercised in selecting the location, number, and type of electrical measurements used to determine the adequacy of cathodicprotection. If tanks are empty, there may be large areas of the bottoms whichare not in contact with the underlying soil. Potential surveys, in this case, maygive misleading information. Tank bottom-to-electrolyte potential readingsmay indicate adequate cathodic protection for the portion of the tank bottomin contact with the soil but when the tank is full and all of the tank bottom is incontact with the soil, protection may be insufficient. Therefore, potentialsurveys should be conducted with an adequate level in the tank to maximizecontact of the tank bottom with the pad material. It is good practice tomark/label the locations on aboveground storage tanks where the referenceelectrode is placed to ensure consistency from survey to survey.

11.1.3 If cathodic protection devices are shut off while working on aboveground storage tanks, the system should be re-energized as soon as possible to avoid corrosion damage during extensive maintenance periods.


11.2 Safety11.2.1 All impressed current systems shall be designed with safety in mind. Care shall be taken to ensure that all cables are protected from physical damage and the possibility of arcing.11.2.2 Rectifiers and junction boxes shall meet regulatory requirements for the specific location and environment in which they are installed. Such locations shall be determined by reviewing local, state, federal, and prevailing industrial codes. Consideration should be given to locating isolating devices, junction boxes, and rectifiers outside of hazardous areas in case sparks or arcs occur during testing.

11.2.3 In order to prevent arcing, care shall be exercised when working on breakout piping attached to tanks with cathodic protection applied. When cathodic protection systems are turned off, sufficient time shall be allowed for depolarization before opening connections. Bonding cables shall be used when parting breakout piping joints. Additional guidance regarding arcing due to static electricity, stray currents or lightening can be obtained from API 2003.


11.3 Cathodic Protection Surveys11.3.1 General11.3.1.1 Prior to energizing a new cathodic protection system, measurements of the native structure-to-soil potential should be made. Immediately after anycathodic protection system is energized or repaired, a survey should beconducted to determine that it operates properly. An initial survey to verifythat it satisfies applicable criteria should be conducted after adequatepolarization has occurred. Polarization to a steady state may take severalmonths after the system is energized. This survey should include one or moreof the following types of measurements:a) polarized structure-to-soil potential;b) anode current;c) native structure-to-soil potentials;d) structure-to-structure potential;e) piping-to-tank isolation if protected separately;f) structure-to-soil potential on adjacent structures;g) continuity of structures if protected as a single structure;h) rectifier DC volts, DC amps, efficiency, and tap settings.


11.3.1.2 Annual cathodic protection surveys are recommended to ensure theeffectiveness of cathodic protection. The electrical measurements used in the survey may include one or more of the measurements listed in 11.3.1.1.

11.3.2 Inspection, Testing, and Maintenance of Cathodic Protection Facilities11.3.2.1 Inspection and tests of cathodic protection facilities should be made to ensure their proper operation and maintenance.11.3.2.2 All sources of impressed current should be checked at intervals not exceeding two months unless specified otherwise by regulation.

Evidence of proper function may be current output, normal power consumption, a signal indicating normal operation, or satisfactory electrical state of the protected structure. A satisfactory comparison between the rectifier operation on a bimonthly basis and the rectifier operation during the annual survey implies the protected status of affected structures is similar. This does not take into account possible effects of foreign current sources.


Evidence of proper function may be; current output, normal power consumption, a signal indicating normal operation, or satisfactory electrical state of the protected structure. A satisfactory comparison between the rectifier operation on a bimonthly

basis and the rectifier operation during the annual survey implies the protected status of affected structures is similar. This does not take into account possible effects of foreign current sources.


11.3.2.3 All impressed current protective facilities should be inspected annually as part of a preventive maintenance program to minimize in-service failure. Inspections should include a check of the general condition, including electrical shorts, ground connections, rectifier meter accuracy, efficiency, and circuit resistance.11.3.2.4 The effectiveness of isolating devices and continuity bonds should be evaluated during the periodic surveys. This can be accomplished by onsite inspection or by evaluating cathodic protection test data.11.3.2.5 The tank bottom should be examined for evidence of corrosion whenever access to the bottom is possible. This may be during repairs ormodifications, or in conjunction with inspections required by API Standard653. Examination for bottom-side corrosion may be achieved by makingcoupon cutouts or by nondestructive methods such as ultrasonic inspectionsor electromagnetic flux leakage.


11.3.2.6 Remedial measures should be taken where periodic tests and inspections indicate that protection is no longer adequate according to applicable criteria. These measures may include the following:

a) repair, replacement, or adjustment of cathodic protection systemcomponents;

b) providing supplementary cathodic protection where additional protection is necessary;

c) repair, replacement, or adjustment of continuity and interference bonds;d) elimination of accidental metallic contacts;e) repair of defective isolation devices;f) resolution of interference currents.


11.4 Cathodic Protection Records11.4.1 Data pertinent to the design, installation, operation, maintenance, and effectiveness of corrosion controlmeasures should be documented in a clear, concise, and workable manner.11.4.2 In determining of the need for cathodic protection, items listed in 5.2 should be recorded.11.4.3 In designing cathodic protection systems, the following should be recorded:a) design and location of isolating devices, test leads and other test facilities, and details of other special corrosioncontrol measures taken;b) results of current requirement tests, where made, and procedures used;c) native structure-to-soil potentials before current is applied;d) results of soil resistivity tests at the site, where they were made, and procedures used;e) name of person conducting surveys.


11.4.4 In installing corrosion control facilities, the following should be recorded.a) Impressed current systems:1) location and date placed in service;2) number, type, size, depth, backfill, and spacing of anodes;3) specifications of rectifier or other energy source;4) interference tests and the parties participating in resolution of any interference problems.b) Galvanic anode systems:1) location and date placed in service;2) number, type, size, depth, backfill, and spacing of anodes unless part of a factory installed system.


11.4.5 A record of surveys, inspections, and tests described in 11.3.1 and 11.3.2 should be maintained to demonstrate that applicable criteria for cathodic protection have been satisfied.

11.4.6 In maintaining corrosion control facilities, the following information should be recorded:

a) repair of rectifiers and other DC power sources;b) repair or replacement of anodes, connections, and cable;c) maintenance, repair, and replacement of coating, isolating devices, test

leads, and other test facilities.

11.4.7 Records sufficient to demonstrate the need for corrosion controlmeasures should be retained as long as the facility involved remains in service. Records related to the effectiveness of cathodic protection should be retained for a period of five years unless a shorter period is specifically allowed by regulation.


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